Just by coincidence, Pretorius and two ghoulish, hired assistants (fugitive murderers) Karl (Dwight Frye) and Ludwig (Ted Billings) enter the mausoleum with lanterns to rob the corpse of a young 19-year-old girl for the madman's life-creating experiments. The grave-robbers Karl and Ludwig fearfully and humorously complain about the setting:

Karl: I can smell the ghosts already.
Ludwig: I never could stand graves.

The Monster hides in the dark shadows and watches the proceedings. After they have opened the coffin to steal the girl's skeleton ("a fresh one"), Karl and Pretorius both make macabre observations:

Karl: Pretty little thing in her way, wasn't she?
Pretorius: I hope her bones are firm.

The two assistants leave the mausoleum after being paid, but Pretorius remains to have a meal and a smoke in the funereal surroundings: "I should wait here for a bit. I rather like this place." Outside, his two assistants depart, complaining bitterly about grave-robbing:

Karl: If there's much more like this, what do you say, pal? We give ourselves up and let 'em hang us.
Ludwig: That goes for me too.
Karl: This is no life for murderers!

Alone inside the crypt for his dinner meal, the deranged Pretorius toasts a Gothic skull that he has placed decoratively on the top of the coffin between lighted candles, laughing unwholesomely to himself for his ghastly joke:

I give you the Monster.

Appropriately, the Monster staggers up to Pretorius, not frightening him in the least. The grave-robbing, scheming scientist befriends the Monster for his own ends, forgetting about his original grave-robbing errand. The Monster is told that a companion is in preparation:

Pretorius: Oh, I thought I was alone. Good evening.
Monster: Smoke? Friend.
Pretorius: Yes, I hope so. Have a cigar. They are my only weakness. [This contradicts his earlier statement to Henry Frankenstein that gin was his only weakness.] (The Monster begins eating voraciously)
Monster: (gesturing) Drink. Good. (Pretorius plies the Monster with wine to get him drunk. The wine is greedily guzzled.) You make man like me?
Pretorius: No. Woman. Friend for you.
Monster: Woman - Friend - Yes - I want Friend, like me.
Pretorius: I think you can be very useful. And you will add a little force to the argument if necessary. Do you know who Henry Frankenstein is and who you are?
Monster: Yes, I know. Made me from dead. I love dead. Hate living.
Pretorius: You're wise in your generation. We must have a long talk, and then I have an important call to make.

The death-loving Monster reacts with pleasure, embraces and speaks to the skull while contemplating his future love object and mate:

Woman - Friend - Wife.

Pretorius reacts perversely with a smirk and leer toward the Monster.

Later, Pretorius re-visits Henry and his new bride - he is vehemently told by Elizabeth Frankenstein that he is "most unwelcome" and that they are leaving. Sniveling and vacillating about what to do, Henry is told about the scientist's preparations for their collaborative experiment. Wracked with wretched pain and anxiety, Henry changes his mind and refuses to cooperate by backing out of their partnership:

Pretorius: All the necessary preparations are made. My part in the experiment is complete. I have created by my method a perfect human brain, already living but dormant. Everything is now ready for you and me to begin our supreme collaboration.
Henry: No, no. Don't tell me of it. I don't want to hear. I've changed my mind. I won't do it!
Pretorius: I expected this.

To persuade Frankenstein to join in his part of the experiment, Pretorius opens the outer door to the Monster, providing a harrowing confrontation between the creature and his creator. Pretorius takes credit for the Monster's ability to talk. The love-sick, fierce-looking Monster endorses Pretorius' mad plans of kidnap and extortion, wishing for a female companion:

Henry: No, not that.
Pretorius: (connivingly) Oh he's quite harmless, except when crossed.
Monster: (pronounced in a deep, gutteral tone tinged with hate) Frank - en - stein.
Pretorius: Yes, there have been developments since he came to me.
Monster: (motioning Henry) Sit down.
Henry: What do you want?
Monster: You know.
Henry: (To Pretorius) This is your work?
Pretorius: Yes.
Henry: I'll have no hand in such a monstrous thing.
Monster: Yes, must.
Henry: Get him out! I won't even discuss it until he's gone.
Pretorius: (To Monster) Go now. (The Monster growls) Go!
Monster: (As he leaves, he turns) Must do it!
Henry: Never! Nothing can make me go on with it. (The Monster angrily grumbles)
Pretorius: (in an aside to the Monster) Now!

To extort Henry into complying and to force his cooperation, Pretorious orders the Monster to kidnap Frankenstein's bride on her wedding day, and then blackmails the entire household: "I charge you as you value your mistress' life to do nothing and say nothing of this episode. I assure you that the Baronness will be safely returned if you will leave everything to me. Nothing, that is, except what he demands." The Monster deposits Elizabeth in a dark, mountain cave. Henry admits defeat to Pretorius after being blackmailed into commencing the creation of a bride for the Monster - he legitimizes his concealed enthusiasm about returning to work: "I admit I'm beaten. But if you can bring her back, I'll do anything that you want." Pretorius promises to return Henry's real bride after the artificial Bride has been created.

Amidst lab equipment in the stone-tower laboratory, now-reopened, work begins as the two mad scientists resume their experiments. But the human heart that they have procured appears unsound:

Pretorius: It is interesting to think, Henry, that once upon a time, we should have been burnt at the stake as wizards for this experiment.
Henry: Doctor, I think the heart is beating! (A drumbeat is heard on the soundtrack.) Look, it's beating, but the rhythm of the beat is uneven.
Pretorius: Increase the saline solution. Is there any life yet?
Henry: No. Not life itself yet...This action only responds when the current is applied.
Pretorius: You must be patient. The human heart is more complex than any other part of the body. Look, the beat is increasing.
Henry: Yes! (after a moment) It stopped.
Pretorius: Shall I increase the current?
Henry: (after indicating a negative response) This heart is useless. I must have another, and it must be sound and young.

In return for a payment of a thousand crowns, Karl is instructed to go to "the accident hospital" for a heart from "a female victim of sudden death." Unbeknownst to Henry and Pretorius, Karl murders a young peasant girl walking in the street to extract the fresh heart from her bosom. [In one of the discarded versions of the film, Karl brings back the heart of Henry's captive bride. How ironic that would have been - the new female Monster would be Henry's Bride twice-over!] When they test the heart, it beats perfectly "just as in life," but Henry is very suspicious about where the transplant organ came from - although he overlooks the fact of the murder:

Karl: It was a very fresh one.
Henry: Where did you get it?
Karl: I gave the gendarme fifty crowns.
Henry: What gendarme?
Karl: It was a ...(after prompting from Pretorius)...police case.
Pretorius: (interjecting) Yes, very sad. Although we can't bother about that now.

Exhausted and distraught, Henry complains about the growling Monster's presence in the laboratory. The Monster grunts commands for him to work:

The Monster: Work!
Henry: (agonizing) Where's Elizabeth? Have you brought her?
The Monster: He wait. I wait.
Henry: I'm exhausted. I must get sleep.
The Monster: Work. Finish. Then sleep.
Henry: I can't work like this. He must go away. Send him away.
Pretorius: I'll settle him for a little while.

No longer needing him, Pretorius callously subdues the impatient Monster with more whiskey (and, in plain view, drugs him with a sedative) to prevent him from interfering. Henry also whines and worries about the safety of his wife Elizabeth. To prove that she is still alive, Pretorius suddenly produces a pre-Bell telephone "electrical machine" so that Henry can speak to her: "Speak - she will hear you and answer." After a short conversation, Pretorius promises: "That is all now. As soon as our work is completed, she will be returned to you." [Imaginative, angled camera close-ups visually emphasize the connectedness of the two characters. In the first tilted, angled shot, Henry looks upwards to the right toward Pretorius. In the next tilted shot, Pretorius looks upwards to the left toward Henry. They are reflective mirror images to each other. The final shot puts both men together, but Pretorius' face is seen framed and separated from Henry's by the opening of a pulley wheel.]

Henry regains his old, 'mad doctor' spirit, forgetting about the risks to Elizabeth's life. The two scientists expectantly discuss with increasing excitement how close they are to beginning their ungodly experiment. [More tilted angle, close-up camera shots alternate back and forth between the two, mirroring them and establishing their bonded relationship. Now Pretorius looks upwards to the right toward Henry. In the next shot, Henry looks upwards to the left toward Pretorius.] They move a large table of surgical instruments closer toward the corpse. [A tilted camera angle makes it appear as if the heavy table is being pushed uphill. A switch in camera angles shows the table coming directly toward the camera on a downhill slant.

The memorable sequence of bringing the Bride to life is unequaled - even the original scene in Frankenstein pales in comparison. While a nighttime, electrical storm rises outside, Frankenstein and Pretorius are overjoyed: "It's going to be a terrific storm!" Amidst weird electrical devices and the sound of thunder, Pretorius removes the sheet covering the bandaged, mummified corpse. Karl is told to proceed to the roof and send up the metal kites - to receive the spark of life in the form of a lightning bolt. Crackling electrical devices are set into motion after Henry returns from the roof. Sparks fly, smoke plumes rise, arcs of electricity jump between devices, switches are thrown, and a spiraling, circular generator collects and harnesses electrical energy. The gurney with the corpse is sent to the roof.

The Monster also stalks up to the roof and kills Karl by hurling him from the top of the stone tower after appearing to be threatened with a lighted torch - this happens just at the climactic moment that lightning strikes one of the kites. After the operating table is lowered back down into the laboratory and the defuser bands are removed from the corpse, one hand stirs - the corpse is imbued with life.

The presentation of the birth of the Monster's Bride (Elsa Lanchester) is stunningly grotesque. When Pretorius removes the bandages from her eyes, two uncomprehending globes stare back. Henry exclaims: "She's alive! Alive!" The two scientists tilt the table to an upright vertical position. The Bride stiffly raises her arms and then collapses. After the bandage-covered Bride has her wrappings removed, indicated by a dissolve, she is viewed in full, chilling splendor. The repellantly-beautiful female wears a flowing white shroud (laboratory smock) and a wild, frizzled fright wig. Streaked with white from lightning charges, her hair stands straight out behind her, making her look dramatically like Queen Nefertiti. Stitches are visible beneath her jaw. Her angular movements are bird-like - her sharp-boned and angular head jerks and darts from one position to another. Appropriately, her white covering could be mistaken for a bridal gown - Pretorius announces:

The Bride of Frankenstein!

Wedding church bells ring on the soundtrack as she is unveiled. Unstable on her feet, the now-living woman wobbles and sways back and forth. The Monster eagerly rushes down into the laboratory to meet and woo his new Bride. She stands back warily. In a heart-breaking parody of typical romantic courtships, he approaches and shyly but hopefully asks:

Friend? Friend?

The ugly creature's clumsy and ludicrous overtures are rejected and repelled by the unreceptive Bride - she recoils and emits a piercing, ear-shattering shriek, one of the most famous screams in screen history, when he reaches out to touch her arm. The Bride pulls away in disgust and seeks shelter behind her creator Frankenstein. The Monster refuses to "stand back" and proceeds to sit next to her. He takes her reptilian, claw-like hand in his own and caresses it, causing her more horrified distaste and another shrewish shriek. Frankenstein pulls her away from her monstrous, abominable counterpart. The Monster is hurt and then angry, declaring with a feeling of worthlessness: "She hate me. Like others."

With a broken, humiliated and rejected heart, the Monster goes beserk, realizing the utter hopelessness of his life and determined to destroy everything around him in a rage. He threatens to pull a lever that will destroy the tower and everything in it:

Henry: (yelling) Look out! The lever!
Pretorius: (warning) Get away from that lever. You'll blow us all to atoms.

Elizabeth escapes from her bonds and races to the tower door where she pleads for Henry to flee. Henry is given a choice - dramatically positioned between life and death. The Monster spares Henry (and Elizabeth) by permitting them to rush to safety outside. However, he pronounces judgment upon Pretorius's perversity:

Elizabeth: Henry! Open the door! Henry!
Henry: Get back! Get back!
Elizabeth: I won't, unless you come!
Henry: (Henry opens the door and hugs Elizabeth.) I can't leave them, I can't.
Monster: Yes, go! You live! Go! (To Pretorius) You stay! We belong dead!

[The original script called for everyone to be destroyed. Changes in the script at the last minute let Henry and Elizabeth live to provide a happy ending, although some of the footage of the destruction of the lab remains that shows Henry's demise in the collapsing laboratory against one of the walls where he is crushed by falling rock and beams.]

As the Monster reaches for and pulls the fateful lever to bring both creatures and creator to extinction, the Bride expels one long, snake-like hiss at him. A tear rolls down the Monster's repulsive face as he summons a fatally-aborted 'honeymoon' night. Explosions begin to rock the stone-tower - rubble from the crumbling foundation buries everyone inside alive. On a nearby hillside, Elizabeth and Henry (who has renounced his mad career once and for all) happily embrace as he offers comforting words to her: "Darling. Darling."