De Witt overhears an encounter between Eve and Bill in her dressing room after her magnificent performance and theatrical debut. The young star 'comes on' to Bill with her big doe eyes, but he doesn't consider himself a "Svengali" and rebuffs her tempting advances:

Bill: ...you did it. With work and patience, you'll be a good actress if that's what you want to be.
Eve: (purring) Is that what you want me to be?
Bill: I'm talking about you and what you want.
Eve: So am I.
Bill: What have I got to do with it?
Eve: Everything.
Bill: Names I've been called, but never Svengali. Good luck.
Eve: Don't run away, Bill.
Bill: From what would I be running?
Eve: You're always after truth on the stage. What about off?
Bill: I'm for it.
Eve: Then face it. I have. Ever since that first night here in this dressing room.
Bill: When I told you what every young actress should know?
Eve: When you told me that whatever I became, it would be because of you...
Bill: Makeup's a little heavy.
Eve: ...and for you.
Bill: You're quite a girl.
Eve: You think?
Bill: I'm in love with Margo. Hadn't you heard?
Eve: You hear all kinds of things.
Bill: I'm only human, rumors to the contrary. And I'm as curious as the next man.
Eve: Find out.
Bill: The only thing - what I go after, I want to go after. I don't want it to come after me. (pause) Don't cry. Just score it as an incomplete forward pass.

After Bill departs and Eve rips her wig from her head, De Witt enters and advises her to quit being so fake, modest and humble:

De Witt: But if I may make a suggestion...I think the time has come for you to shed some of your humility. It is just as false not to blow your horn at all as it is to blow it too loudly.
Eve: I don't think I have anything to sound off about.
De Witt: We all come into this world with our little egos, equipped with individual horns. Now if we don't blow them, who else will?
Eve: Even so, one pretty good performance by an understudy - it'll be forgotten by tomorrow.
De Witt: It needn't be.

Eve continues to exploit De Witt to the fullest - he invites her to dinner. As she changes her clothes, he gathers information from her for his column. There is so much he wants to know about her background, so he slyly asks about the start of her idolatrous emulation of Margo: "I've heard your story in bits and pieces - your home in Wisconsin, your tragic marriage, your fanatical attachment to Margo. It started in San Francisco, didn't it?" Eve remembers that she was first dazzled by Margo on the stage at the Shubert Theatre in San Francisco. A turning point in the film, De Witt crowns the Shubert Theatre as "an oasis of civilization in the California desert," a "fine old theatre...full of tradition, untouched by the earthquake, or should I say, fire." When De Witt presumes they will have a "special night" together, Eve seductively implores:

Eve: You take charge.
De Witt: I believe I will.

The next morning's papers, thanks to De Witt's engineering, carry articles praising Eve's performance as Margo's understudy. De Witt and Eve are at the Twenty-One Restaurant to meet a movie talent scout ("a sun-burned eager beaver") for lunch the same day, although "Eve has no intention of going to Hollywood." Karen, who is to meet Margo at the same restaurant for lunch, is given Addison's "poison pen" review in the paper to read.

The insidious column also angers Margo, who reads the plaudits for Eve's youthful role and a scathing interview in which Eve makes unflattering statements about aging actresses who play inappropriate, younger roles:

And so my hat, which has lo these many seasons become more firmly rooted about my ears, is lifted to Miss Harrington. I am once more available for dancing in the streets and shouting from the housetops...Miss Harrington had much to tell and these columns shall report her faithfully about the lamentable practice in our theatre of permitting, shall we say, mature actresses to continue playing roles requiring a youth and vigor which they retain but a dim memory...about the understandable reluctance on the part of our entrenched first ladies of the stage to encourage, shall we say, younger actresses about Miss Harrington's own long and supported struggle for opportunity.

She is incensed about Eve's role in gathering critics (especially De Witt) to attend her understudy performance, and by her aggressive rise to stardom: "The little witch must have sent out Indian runners, snatching critics out of bars and steam rooms and museums, or wherever they holed up. Well, she won't get away with it, nor will Addison De Witt and his poison pen. If Equity or my lawyer can't or won't do anything about it, I shall personally stuff that pathetic little lost lamb down Mr. De Witt's ugly throat." Bill arrives and sympathizes with Margo's reaction, calling De Witt's writing "that piece of filth." They are reconciled to each other.

In their own apartment with Karen, Lloyd inaccurately blames Addison for being behind Eve's ambitious quest: "It's Addison from start to finish. It drips with his brand of venom. Taking advantage of a kid like that, twisting her words, making her say what he wanted her to say." [Addison's name, similar to the name of the 'adder' snake comes to mind, as he tempts Eve - recalling the Garden of Eden.] Eve has begun to win over Karen's husband Lloyd - she has convinced him that she would be "fine for the part" - a starring role in his new play Footsteps on the Ceiling, to be put into production right away, playing the "younger" character of Cora (a role that Margo was originally to play). This could occur if Margo could be talked into going on tour with Aged in Wood.

Karen suspects that Eve will stop at nothing to get the part: "Eve would ask Abbott to give her Costello." When he takes Eve's side, Lloyd detects bitter cynicism in Karen's voice when she denounces Eve as a "contemptible little worm."

Lloyd: For once to write something and have it realized completely. For once not to compromise.
Karen: Lloyd Richards! You are not to consider giving that contemptible little worm the part of Cora.
Lloyd: Now just a minute.
Karen: Margo Channing's not been exactly a compromise all these years. Why, half the playwrights in the world would give their shirts for that particular compromise.
Lloyd: Now just a minute.
Karen: It strikes me that Eve's disloyalty and ingratitude must be contagious.
Lloyd: All this fuss and hysteria because an impulsive kid got carried away by excitement and the conniving of a professional manure-slinger named De Witt. She apologized, didn't she?
Karen: On her knees, I've no doubt. Very touching. Very Academy of Dramatic Arts.
Lloyd: That bitter cynicism of yours is something you've acquired since you left Radcliffe.
Karen: That cynicism you refer to I acquired the day I discovered I was different from little boys.

Later that night in The Cub Room after the evening's theatrical performance (after which Eve gave her notice as Margo's understudy), the two couples (Margo and Bill, and Karen and Lloyd) meet for a bottle of wine to celebrate a special occasion. Bill now announces with a toast his proposal of marriage to Margo - the next day at 10 at City Hall they will acquire a marriage license:

Bill: The so-called art of acting is not one for which I have a particularly-high regard...But you may quote me as follows. Quote: 'Tonight, Miss Margo Channing gave a performance in your cockamamie play the like of which I have never seen before and expect rarely to see again.' Unquote....I shall propose the toast. Without wit. With all my heart. To Margo. To my bride-to-be.
Margo: Glory, Hallelujah.

Asked by Karen what she will wear to her wedding, Margo replies: "Something simple. A fur coat over a nightgown." Eve, who just happens to be in the same restaurant dining with De Witt, sends a note to Karen, urgently requesting to speak to her in the ladies room. In her inimitable manner, Margo asks the waiter for more champagne:

Margo: Encore du champagne.
Waiter: More champagne, Miss Channing?
Margo: That's what I said, bub.

Everyone is curious about Eve and Margo questions "what's going on in that feverish little brain waiting in there." When Eve speaks to Karen in the ladies room, she first disclaims, over-dramatically, the hurtful statements she made in Addison's column although she accepts the responsibility and the disgrace. She also describes how she has "been told off in no uncertain terms all over town" when loyal Margo Channing supporters came to the aging actress' defense and caused a backlash against her. Then in a quick turnaround, she blackmails Karen into pressuring Lloyd to use her in the lead youthful role in her husband's new play: "If you told him so, he'd give me the part. He said he would...It's my part now...Cora is my part. You've got to tell Lloyd it's for me." She threatens Karen with divulging the "perfectly harmless joke" played on Margo, accentuating the fact that "Addison could make quite a thing of it - imagine how snide and vicious he could get and still tell nothing but the truth." She effectively blackmails Karen to get her the acting part as a "simple exchange of favors":

If you told him (Lloyd) so, he'd give me the part. He said he would...It's my part now...Cora is my part. You've got to tell Lloyd it's for me...Addison wants me to play it...Addison knows how Margo happened to miss that performance, how I happened to know she'd miss it in time to call him and notify every paper in town...If I play Cora, Addison will never tell what happened, in or out of print. A simple exchange of favors. I'm so happy I can do something for you at long last. Your friendship with Margo - your deep, close friendship. What would happen to it, do you think, if she knew the cheap trick you played on her for my benefit? You and Lloyd. How long, even in the theatre, before people forgot what happened and trusted you again? No, it would be so much easier for everyone concerned if I would play Cora. So much better theatre too.

It is now obvious that Margo's view of the young actress was correct. Many fall victim to Eve's Machiavellian, cold-blooded, destructive plans to further her own ends. Karen is astounded: "You'd do all that just for a part in a play." Eve replies, predictably: "I'd do much more for a part that good" before returning to her seat at Addison De Witt's table.

To Karen's surprise - before she can completely relate her conversation with Eve - Margo confides to Lloyd that she doesn't want to play the part of Cora in the new play Footsteps on the Ceiling, now that she is becoming a "married lady":

Never have I been so happy...I'm forgiving tonight, even Eve, I forgive Eve...Do you know what I'm going to be?...A married lady...No more make believe off stage or on. Remember, Lloyd? I mean it now...I don't want to play Cora...It isn't the part. It's a great part in a fine play. But not for me anymore. Not for a four-square, upright, downright, forthright married lady...It means I finally got a life to live. I don't have to play parts I'm too old for, just because I've got nothing to do with my nights.

Margo retires from the intrigues of the stage in favor of marriage. Relieved, Karen doesn't have to convince her husband to cast Eve instead of Margo after all. Yet, in voice-over, Karen fears she's losing her husband with the rift growing daily with him due to his association with the conniving Eve. Fighting ("always over some business for Eve") ensues in rehearsals with Eve playing the part of Cora. Eve breeds dissension between Lloyd and Bill ("somehow Eve kept them going"):

Lloyd never got around somehow to asking whether it was all right with me for Eve to play Cora. Bill, oddly enough, refused to direct the play at first - with Eve in it. Lloyd and Max finally won him over. Margo never came to rehearsal. Too much to do around the house, she said. I'd never known Bill and Lloyd to fight as bitterly and often and always over some business for Eve, or a move, or the way she read a speech. But then I'd never known Lloyd to meddle as much with Bill's directing, as far as it affected Eve, that is. Somehow Eve kept them going. Bill stuck it out. Lloyd seemed happy. And I thought it might be best if I skipped rehearsals from then on. It seemed to me I had known always that it would happen, and here it was. I felt helpless, that helplessness you feel when you have no talent to offer - outside of loving your husband. How could I compete? Everything Lloyd loved about me, he had gotten used to long ago.

Late one night, Karen answers a phone call for her husband from Eve Harrington's worried neighbor, reporting that "she isn't well, she's been crying all night, and she's hysterical and she doesn't want a doctor." Lloyd quickly volunteers to immediately come over and attend to Eve. With a worried look on her face, Karen senses further disruption in her marriage. Her suspicions prove to be correct. The camera pans from the neighbor to the right where enchantress Eve sits on the stairs - she is behind the set-up to call Lloyd and steal him away from his wife in the middle of the night.

The new play's out-of-town opening ("Max Fabian presents Footsteps on the Ceiling, a new play by Lloyd Richards") is scheduled for the Shubert Theater in New Haven, Connecticut ("It is here that managers have what are called out-of-town openings which are openings for New Yorkers who want to go out of town"). On the day of the opening, Eve encounters De Witt outside the Taft Hotel next to the theater and they walk along as De Witt predicts that Eve will become a major star after the triumphant opening and her playing of the lead role in Lloyd's play. At her hotel room door, he questions about how she can calmly nap so easily in the afternoon before her debut show:

Eve: What a heavenly day!
De Witt: D-Day.
Eve: Just like it.
De Witt: And tomorrow morning, you will have won your beachhead on the shores of immortality.
Eve: Stop rehearsing your column. Isn't it strange, Addison? I thought I'd be panic-stricken, want to run away or something. Instead, I can't wait for tonight to come, to come and go.
De Witt: Are you that sure of tomorrow?
Eve: Aren't you?
De Witt: Frankly, yes.
Eve: It will be a night to remember. It will bring me everything I've ever wanted. The end of an old road. The beginning of a new one.
De Witt: All paved with diamonds and gold?
Eve: You know me better than that.
De Witt: It's paved with what, then?
Eve: Stars...Plenty of time for a nice long nap. We rehearsed most of last night.
De Witt: You could sleep now, couldn't you?
Eve: Why not?
De Witt: The mark of a true killer. Sleep tight, rest easy, and come out fighting.
Eve: Why did you call me a killer?
De Witt: Oh, did I say killer? I meant champion. I get my boxing terms mixed.

In her expensive suite, an experienced De Witt clearly sees Eve's duplicitous and manipulative nature, similar to his own selfishness. He is one of the few who immediately recognized her cold and calculating heart and saw through her scheming charade from the very beginning. He is not too startled to learn that Eve has designs on taking Lloyd from Karen for her own purposes. According to Eve's way of thinking, Lloyd ("commercially the most successful playwright in America...and artistically the most promising") is planning to marry her. With Lloyd serving as her husband and playwright, Eve would have the pick of parts in her future as he would write plays specifically for her:

Lloyd Richards. He's going to leave Karen. We're going to be married...Lloyd loves me, I love him...I'm in love with Lloyd...Oh Addison, won't it be just perfect? Lloyd and I - there's no telling how far we can go. He'll write great plays for me, I'll make them great.

De Witt objects to her "unholy alliance" with Lloyd and confronts her with the fact that she had never been the innocent Eve Harrington - she was using Lloyd to get "a run-of-the-play contract." He is angered that she is playing him off against Lloyd, a 'divide-and-conquer' weapon in her arsenal. He wonders whether he is being made a fool too, like everyone else:

Eve: (starry-eyed) The setting wasn't romantic, but Lloyd was. He woke me up at three o'clock in the morning banging on my door. He couldn't sleep, he said. He'd left Karen. Couldn't go on with the play or anything else until I promised to marry him. We sat and talked until it was light. He never went home.
De Witt: You 'sat and talked' until it was light?
Eve: We 'sat and talked' Addison. I want a run-of-the-play contract.
De Witt: There never was and there never will be another like you...(rising) What do you take me for?
Eve: I don't know that I'd take you for anything.
De Witt: Is it possible, even conceivable, that you've confused me with that gang of backward children you play tricks on? That you have the same contempt for me as you have for them?...Look closely, Eve. It's time you did. I am Addison De Witt. I am nobody's fool. Least of all - yours.
Eve: I never intended you to be.
De Witt: Yes you did and you still do...It's important right now that we talk - killer to killer.
Eve: Champion to champion.
De Witt: Not with me, you're no champion. You're stepping way up in class.
Eve: Addison, will you please say what you have to say, plainly and distinctly, and then get out so I can take my nap.
De Witt: Very well. Plainly and distinctly...Lloyd may leave Karen, but he will not leave Karen for you.
Eve: What do you mean by that?
De Witt: More plainly and more distinctly? I have not come to New Haven to see the play, discuss your dreams, or pull the ivy from the walls of Yale. I've come here to tell you that you will not marry Lloyd or anyone else for that matter because I will not permit it.
Eve: What have you got to do with it?
De Witt: Everything, because after tonight, you will belong to me.

Eve has underestimated De Witt's own ambitions. He has his own designs on Eve, hoping to have her all to himself as his mistress - this is the price Eve must pay. When she chuckles at the thought of belonging to him ("Belong to you? That sounds medieval, something out of an old melodrama"), he slaps her sharply across the face, insulted: "Now remember as long as you live, never to laugh at me. At anything or anyone else, but never at me."

In a dramatic confrontation, he demolishes her manufactured sob story she told at the beginning of her idolization of Margo: "To begin with, your name is not Eve Harrington. It's Gertrude Slescynski." He knows all about her real, sordid background: her parents were poor and hadn't heard from her for three years; she was paid $500 to leave her brewery job and get out of town after an alleged scandalous affair with the boss; there was no pilot husband named Eddie who was killed in the war; and she was never married. De Witt reveals the crowning lie that would expose her:

De Witt: San Francisco has no Shubert Theater. You've never been to San Francisco! That was a stupid lie, easy to expose, not worthy of you.
Eve: I had to get in to meet Margo! I had to say something, be somebody, make her like me!

Although Margo at first liked her, Eve betrayed her trust by "trying to take Bill away." De Witt overheard her attempt to seduce Bill after her understudy performance. And she also used De Witt's name and column to blackmail Karen into getting her the part of Cora, and then lied to the critic about it. De Witt gloats: "I had lunch with Karen not three hours ago. As always with women who try to find out things, she told more than she learned. Now do you want to change your story about Lloyd beating at your door the other night?"

De Witt agrees to settle for her, even though she has ruthlessly betrayed her friends and lied about her past - undoubtedly, she is an ambitious, shameless and opportunistic actress, without feelings or scruples.

That I should want you at all suddenly strikes me as the height of improbability, but that, in itself, is probably the reason. You're an improbable person, Eve, and so am I. We have that in common. Also a contempt for humanity, an inability to love and be loved, insatiable ambition - and talent. We deserve each other...and you realize and you agree how completely you belong to me?

Devastated, Eve listlessly nods agreement that she belongs to him - she has suddenly become the victim of her own trap, unable to escape and still become a success. Eve protests that she couldn't possibly go on stage that night. De Witt thinks otherwise:

Couldn't go on! You'll give the performance of your life.

The film dissolves back to the awards ceremony, where Eve has just received the Sarah Siddons Best Actress of the Year trophy. "And she gave the performance of her life. And it was a night to remember that night." Eve gives credit for her acting to her "friends in the theatre and to the theatre itself" with proper humility and gratitude - her words ring with hypocritical emptiness. "In good conscience, I must give credit where credit is due" - to those in the audience who have helped her the most - Max, Karen, Margo, Bill and Lloyd - are also those she has used, discarded, and hurt the most. She tells the audience, during "the happiest night of my life" that although she is leaving for Hollywood to make a film, her heart will remain in the theater on Broadway - "three thousand miles are too far to be away from one's heart." And she will be back to reclaim her heart soon, if they want her back.

After the ceremony, Margo 'congratulates' Eve:

Nice speech, Eve. But I wouldn't worry too much about your heart. You can always put that award where your heart ought to be.

Eve decides to forgo a celebratory party at Max's, and gives the award to De Witt to take there in her place. Tired, Eve is dropped off from their shared taxi ride at her hotel/apartment and is startled by a breathless fan club president from Brooklyn, a pretty, star-struck teenager named Phoebe (Barbara Bates) who has fallen asleep in a chair in her suite. At first, Eve is ready to call the authorities, but then flattered by the attention, sees a striking resemblance to herself in the young protege - she's a little 'Eve' of her own. Phoebe is writing a report on her idol: "About how you live, what kind of clothes you wear, what kind of perfume and books, things like that." She aspires to be like some of the Brooklynites who became famous in Hollywood:

Well, lots of actresses come from Brooklyn. Barbara Stanwyck and Susan Hayward. Of course, they're just movie stars.

When De Witt delivers the award statuette that had been left in the taxi cab, Phoebe answers the door, and recognizes Addison De Witt (revealing she is as knowledgeable as Eve was earlier in the film). [De Witt presumes, probably accurately, that "Phoebe" is only a stage name.] She takes the award from his hands:

Phoebe: I call myself Phoebe.
De Witt: And why not? Tell me, Phoebe, do you want someday to have an award like that of your own?
Phoebe: More than anything else in the world.
De Witt: Then you must ask Miss Harrington how to get one. Miss Harrington knows all about it.

In an ironic, but pungent ending or postscript, the film audience realizes that it won't be long before Phoebe, like Eve, will be rising the ladder of success at any cost. Eve will be conned in much the same way that Margo was earlier. By an offscreen Eve, Phoebe is asked:

Eve: Who was it?
Phoebe: Just a taxi driver, Miss Harrington. You left your award in his cab and he brought it back.
Eve: Oh. Put it on one of the trunks, will you? I want to pack it.
Phoebe: Sure, Miss Harrington.

Taking the award to Eve's bedroom, Phoebe sets the award on a trunk, but then she sees Eve's glittering outer coat on the bed. She hesitates and then quietly puts it on and clutches the award to her breast in front of a large four-mirrored cheval - one in which Eve would admire herself. Gracefully and with grave dignity, Phoebe poses before the mirror that provides infinite reflections, representing the thousands of Eve Harringtons out there. She steps forward and bows, again and again and again, acknowledging imaginary applause from an audience during a curtain call. Phoebe represents the new wave of thousands of ingenue actresses ready to replace the aging, over-age 40 leading ladies who have reached their upper limit during their brief 'lives' as actresses. The cycle of stardom repeats itself in cynical fashion - for every star, there is someone younger and more ambitious in the wings.