When Mortimer curiously pries open the window seat, he discovers a new body there: "E-gods, there's another one!" Demanding an explanation from Aunt Abby, he learns that the corpse is "a total stranger" to her:

Abby: Who can that be?
Mortimer: Are you trying to tell me you've never seen that man before?
Abby: I certainly am. Well, this is a fine how-do-you-do. It's getting so anybody thinks he can walk in this house.
Mortimer: Now you look here, Aunt Abby. Don't you try to get out of this. That's another of your gentlemen - and you know it!
Abby: Mortimer - how can you say such a thing? That man's an imposter. And if he came here to be buried in our cellar, he's mistaken...
Mortimer: Aunt Abby, how can I believe you? There are twelve men down in the cellar and you admit you poisoned them.
Abby: Yes, I did. But you don't think I'd stoop to telling a fib!

With sudden insight and a goofy grin, Mortimer realizes that the strange corpse ("cold companion") in the window seat has been murdered by Jonathan - he kisses Aunt Abby on the forehead and offers her an apology for being accusatory. During his nightly beat, Officer O'Hara arrives and to keep him in the house, Mortimer agrees to offer his dramatic critic's expertise by discussing the ambitious "playwright's" ideas while he enjoys some wedding cake in the kitchen - the officer complains: "I get wonderful ideas but I can't spell 'em."

Before he joins O'Hara in the kitchen, Mortimer offers Jonathan a last safe chance to leave with the body during the diversion: "Because if you don't leave here in five minutes, I'll bring in Officer O'Hara and introduce him to Mr. Spinalzo." However, Jonathan is determined to remain in the Brewster house (and use it as a front for criminal activities), dump Mr. Spinalzo's body in the bay, and eventually murder his own brother: "We've got a wonderful set-up here. We can make a fortune. Two old ladies as a front. Only Mortimer stands in our way. I never did like Mortimer." To their advantageous surprise, Einstein and Jonathan find Mr. Hoskins' body in the cellar grave - and they use it as a bargaining chip against Mortimer:

If you tell O'Hara what's in the window seat, I'll tell him what's down in the cellar...There's an elderly gentleman down there who seems to be very dead.

Forced to change his tune, Mortimer hurriedly coaxes O'Hara out the door, and promises to discuss his "great play" with him in the backroom of Kelly's. And then he leaves to locate a doctor to evaluate and confirm Teddy's mental illness.

When Einstein and Jonathan are about to carry Mr. Spinalzo down to the cellar for disposal, the two aunts openly admit that they "know about what's down there...(Mr. Hoskins) He's one of our gentlemen...And we won't have any strangers buried in our cellar...Mr. Hoskins is no stranger." They confess to having twelve graves in the overcrowded space that must be reserved for their future charity acts:

Abby: That leaves very little room and we're going to need it.
Jonathan: You mean that you and Aunt Martha have murdered twelve...
Abby: Murdered? Certainly not. It's one of our charities.
Martha: Why, what we've been doing is a mercy.
Abby: So you just take your Mr. Spinalzo out of here.

Jonathan competitively boasts of accumulating thirteen victims to his credit, but is corrected by his delighted, weaselly assistant:

Einstein: You got twelve, they got twelve.
Jonathan: I've got thirteen. (He grabs Einstein's necktie)
Einstein: No, Johnny, twelve. Don't brag.
Jonathan: Thirteen. There's Mr. Spinalzo, and the first one in London, two in Johannesburg, one in Sydney, one in Melbourne, two in San Francisco, one in Phoenix, Arizona.
Einstein: Phoenix?
Jonathan: The filling station.
Einstein: Filling station? Oh! (He remembers slitting the man's throat.) Yes.
Jonathan: Three in Chicago, and one in South Bend. That makes thirteen.
Einstein: You cannot count the one in South Bend. He died of pneumonia.
Jonathan: He wouldn't have died of pneumonia if I hadn't shot him.
Einstein: No, no, Johnny, you cannot count him. You got twelve, they got twelve. The old ladies is just as good as you are.

To better his aunts with one more murder, a jealous Jonathan vows that Mortimer may be his thirteenth victim: "That's easily taken care of. All I need is one more. That's all. Just one more. And I've a pretty good idea who it is."

Mortimer returns with a doubting Dr. Gilchrist (Chester Clute) in tow to conduct a medical examination on Teddy - a prerequisite for incarceration: "I know Teddy blows bugles, but I can't commit a man to an institution just on that." To avoid having the doctor witness the burial ceremony for Mr. Hoskins, Mortimer parks Gilchrist in the nearby cemetery. The taxi-cab driver reminds Mortimer of his mounting tab on the meter as he rushes by: "$22.50." Thinking that he is referring to the price of his suit, Mortimer responds: "Oh. Yes. Looks good on you." When Mortimer jogs out of the house with Teddy a moment later, the cabbie adds: "Hey you! Five more bucks and you'll own it." Mortimer yells back: "No thanks. It wouldn't fit me."

Teddy is introduced to the doctor in the cemetery, and he presumes him to be "Dr. Livingstone" - then he leads the confused physician on a tour of "Arlington." During the interlude, Mortimer pauses to sit on a headstone [one of the tombstones behind him reads "Archie Leach" - Grant's real name]. He speaks to his bewildered new bride at her window. Already reluctant about matrimony, family responsibility and child-rearing due to his heritage (a homicidally-deranged family), Mortimer tells her that he fears that a streak of madness runs deep within his family - and that the Brewster residence is an insane asylum:

Mortimer: Darling, I love you so much I can't go through with our marriage.
Elaine: (drawing back) Have you suddenly gone crazy?
Mortimer: No, no, I don't think so. It's only a matter of time. Look, look darling. You wouldn't want to have children with three heads, would you? I mean, you wouldn't want to set up housekeeping in a padded cell. Oh, it would be bad.
Elaine: What are you talking about?
Mortimer: Well, I don't quite know, Elaine. Look, I probably should have told you this before, but you see, well, Insanity runs in my family. It practically gallops.

Good-naturedly, she unconvincingly tries to reassure him:

Elaine: All this doesn't prove a thing. Look at your aunts. They're Brewsters, aren't they? And the sanest, sweetest people I've ever known.
Mortimer: Well, even they have their peculiarities.
Elaine: What of it? So your family's crazy. So you're crazy. That's the way I want you. That's the way I love you. I'm crazy too, but kiss me.

Dr. Gilchrist tells Mortimer that he is easily persuaded and convinced to "commit him to anyplace" after a short conversation with Teddy: "I've just been appointed Ambassador to Bolivia." Ignored and spiteful, Elaine slams the window down on Mortimer's hand when he stops kissing her to speak to the doctor and have him sign the papers. To carry through on his homicidal plan, Jonathan promises a sleepy Einstein: "Tonight, we are taking care of Mortimer." His idea is to use a "special" slow method of torture called the "Melbourne method" - an excruciatingly painful technique that causes Einstein to quiver: "Not the Melbourne method, please! Two hours!" Mortimer fools Teddy into signing the commitment document by calling it a "secret proclamation." As he descends the stairs with a clinging, snivelling Einstein mumbling to beg him to heed his warning about Jonathan's madness, he bids him to speak louder: "Will you stop underplaying? I can't hear you."

As he sits in a dining room chair, Mortimer stage-directs the actions of the plot of a 'terrible' play currently at the Garrick Theatre: "There's a man...he knows he's in a house with murderers so he ought to know he is in danger. He's even been warned to get out of the house - and does he go?...No, he stays. This fellow doesn't even have sense enough to be scared...He sits down...So there he is, all waiting to be trussed up and gagged...The silly chump sits down with his back to the murderer. All he has to do is look around, but does he?...The big chump sits there. This fellow was supposed to be bright...He sits there waiting to be tied up and gagged (laughter) - the big dope." The hapless Mortimer does a quizzical double-take at the camera when the curtain cord and gag are administered to him and he has become the central character of the play. By candlelight, Jonathan opens up a black case to reveal dozens of shiny implements of torture, and suggests that the pain will be terribly difficult to endure: "The more you struggle, Mortimer, the more you strangle yourself. Later on, you may consider that a blessing." Jonathan dons latex gloves for the "artistic achievement" about to be dramatically performed for the "very distinguished" critic.

Mortimer's eyes light up with hope and he smiles under his gag when Einstein pours glasses of elderberry wine for them to drink before they operate. Momentarily, he cringes that they will offer him a glass, but he is relieved that they only pause to toast him for bringing them back to Brooklyn: "To my dear dead brother." Teddy's bugle-blowing from the top of the stairs startles them and they drop their glasses to the floor. The commotion also summons Officer O'Hara to investigate complaints about loud noises from the disturbed neighbors. O'Hara reprimands Mortimer, still gagged and bound, for standing him up for over an hour. Oblivious to his helplessness, the officer relates the lengthy plot of his play to his speechless, captive audience. Two other officers also arrive to report an overdue O'Hara to the police station: "Tell the lieutenant he can call off the big manhunt. We found him - at the Brewster house." Jonathan mistakenly thinks he has been turned in, so he admits his own guilt: "All right, you've got me," and then confesses to the skeptical cops that thirteen bodies are buried in the cellar. O'Hara's comment that Jonathan "looks like Boris Karloff" sends him into a crazed frenzy. While a large scuffle ensues between them, Mortimer sits calmly on the stairs, smokes a cigarette, and mutters to himself: "All I did was cross the bridge and I was in Brooklyn. Amazing."

When Lieutenant Rooney (James Gleason) arrives, he recognizes Jonathan (who fits the "looked like Karloff" description) - as a "wanted" man: "(Jonathan) escaped from the prison for the criminally insane and he's a lifer." O'Hara is suspended by the lieutenant for disregarding his duties. And when Teddy ("President Roosevelt") joins the crowd downstairs and confirms that there are thirteen bodies ("yellow fever victims") buried in the basement, they don't believe him. In fact, they take his statements as evidence that he is certifiably crazy. Mortimer displays the papers that will send Teddy to the sanitarium that night, and Witherspoon arrives to transport Teddy to the Happydale "rest home" - using the cab already parked outfront. The cabbie retorts: "I knew this would end up in a nuthouse." One final detail is required - Teddy must re-sign the documents with his own name, so Mortimer uses convoluted logic to persuade him:

Mortimer: The name Brewster is code for Roosevelt.
Teddy: Code for Roosevelt?
Mortimer: Yes, don't you see? Take the name Brewster, take away the B, and what have you got?
Teddy: Rooster!
Mortimer: Uh-huh. And what does a rooster do?
Teddy: Crows.
Mortimer: It crows. And where do you hunt in Africa?
Teddy: On the veldt.
Mortimer: There you are: crows - veldt. [Roosevelt]
Teddy: Ingenious! My compliments to the boys in the code department.

Even Mr. Witherspoon pitches his play idea to Mortimer - "a dramatization really of many of the little incidents that have happened there at Happydale." Teddy is eager to leave for Happydale, believing that his term of office is over and he is taking a hunting trip to Africa with Witherspoon as his guide. The two Brewster sisters are concerned that Teddy is being committed, and offer to join him by also signing papers: "If he's going, we're going too." Although Witherspoon objects that they are "sane people," eyes pop out when the aunts admit to the hard-boiled Lieutenant Rooney that "there are thirteen bodies buried in our cellar." Mortimer side-steps confirming their sensational statement by turning crazy and "strange" - he blows Teddy's bugle and charges up the stairs, exaggerating their outrageous claims: "Certainly there are thirteen bodies down in the cellar and there are hundreds more up in the attic...!" While the two aunts are signing their own commitment papers, Dr. Einstein is halted from sneaking out of the house and asked to provide his compulsory physician's signature. As Einstein turns to leave, Rooney receives a description of Jonathan's accomplice over the phone:

About 40, 5 foot 3, hundred forty pounds, pop-eyes, talks with a German accent, poses as a doctor.

The positive identification, amazingly, isn't linked to Einstein, and he quickly escapes from the house.

The major punch line of the film occurs when the neurotic Mortimer learns that he is not a tainted Brewster, after he signs his relatives' commitment papers as next-of-kin and while Witherspoon is packing Teddy's duffel for the trip. When his aunts take him aside, Mortimer is elated at discovering that he is not legally the "next-of-kin" but adopted (and therefore free of inherited insanity that is "contagious" and passed down within his family):

You're not really a Brewster...Your mother came to us as a cook. And you were born about three months afterwards. And she was such a sweet woman and such a good cook. We didn't want to lose her so brother married her. Your real father was a cook too. He was a chef on a tramp steamer.

His concern to responsibly commit his family (and prevent any further murders) is rewarded with the fact that he is told that he is the family's bastard! Overjoyed and half-fainting, he yells out the window that he is sane - and can be properly united with his new bride:

Elaine, Elaine, Where are you? Can you hear me? I'm not really a Brewster. I'm a son of a sea cook!

[Note: The original line in the film to express his delight to her at discovering he isn't a Brewster had been: "Elaine! Did you hear? Do you understand? Darling, I'm a bastard!" but the line was censored and changed.]

Potentially ruining Mortimer's whole scheme, Elaine emerges from the basement with the news that there are "thirteen bodies down there!" He silences her screams with a non-stop smooch, carries her from the house while engaged in a lip-lock, and repeats the last phrase another time to the cabbie ("I'm not a Brewster. I'm a son of a sea cook"). He "chaaarrrggges" away with Elaine over his shoulder for their honeymoon at Niagara Falls. The cabbie responds in kind: "I'm not a cab driver. I'm a coffee pot."

[Note: The play's final scene of the administering of elderberry wine by the two sweet aunts to the self-confessed lonely and unhappy Mr. Witherspoon is eliminated from the film.]