A young man about to be married discovers the two aunts who raised him have been poisoning lonely old men.

After overcoming an attack of pre-nuptial nerves, Mortimer Brewster, New York theatrical critic, confirmed bachelor and author of millions of words against marriage, weds Elaine Harper, a minister's daughter, on Halloween Day. On their way to their Niagara Falls honeymoon, Mortimer and Elaine stop in Brooklyn, where Elaine's father lives next to Mortimer's two maiden aunts and uncle. While Elaine breaks the news of her marriage to her conservative father, Mortimer drops in on his aunts, Abby and Martha. In his aunts' living room, Mortimer searches for notes on his latest book, Mind Over Matrimony , and discovers a corpse in his aunts' window seat. Immediately Mortimer assumes that his deranged uncle, who believes himself to be Teddy Roosevelt, is responsible. To his horror, however, Abby and Martha calmly take credit for murdering Mr. Hoskins and later confess to killing not only Mr. Hoskins but eleven other men, all of whom are now buried in the cellar. Stricken by the story of his aunts' murderous past, which began when an elderly visitor suffered a fatal heart attack in their parlor and inspired them with his peaceful repose, Mortimer tries to point out the error of their killing ways. Instead, Abby and Martha insist that luring lonely men into their home with a "room for rent" sign and serving them elderberry wine laced with arsenic and other poisons is a charitable service. While Mortimer frantically tries to have his uncle, whose lunacy is well-known in Brooklyn, committed to a sanitarium as a means of clearing his aunts of any future blame, Gibbs, another would-be renter, arrives at the house. After preventing Gibbs from taking the fatal sip of elderberry wine, Mortimer leaves to see Judge Cullman, whose signature he needs for "Teddy's" commitment papers. In his absence, Mortimer's criminally insane brother Jonathan, recently escaped from an Indiana asylum, and Dr. Einstein, a drunken underworld plastic surgeon, show up unexpectedly at the house. Unknown to the aunts, Jonathan, whose face Dr. Einstein accidentally altered to resemble Boris Karloff's, has brought along his own murder victim, Mr. Spenalzo. Against his sisters' wishes, Teddy, who has been told by Abby and Martha that Mr. Hoskins, like their other victims, is a yellow fever casualty and must be buried immediately in a "lock" of the "Panama Canal," invites Dr. Einstein to inspect his newly dug hole in the cellar. Dr. Einstein concludes that the "lock" would be an ideal resting place for Mr. Spenalzo, and after Jonathan forces his aunts to retire early, the two criminals move Mr. Spenalzo into the living room, just as Teddy carries Mr. Hoskins to the cellar. Before Jonathan and Dr. Einstein are able to get Mr. Spenalzo into the basement, however, Elaine shows up, forcing them to deposit the corpse in the now vacant window seat. The ever paranoid Jonathan then tries to drag Elaine to the cellar but is stopped by the return of Mortimer. Oblivious to Elaine's distress over Jonathan, Mortimer sends her back home, then after failing to intimidate Jonathan into leaving, finds Mr. Spenalzo's body in the window seat. Although Mortimer first accuses Abby of the deed, Jonathan reveals himself as the culprit when he rushes to sit on the window seat as his aunts are about to open it. Before Mortimer is able to act on his discovery, O'Hara, the new neighborhood policeman, arrives at the door. On the promise that he will discuss O'Hara's autobiographical play with him later that night, Mortimer rids himself of the policeman, but then is confronted by Dr. Einstein, who, while disposing of Mr. Spenalzo's body, stumbled on Mr. Hoskins' corpse in his cellar grave. After Jonathan and his aunts argue about who has the more impressive murder record, Mortimer obtains the second needed signature for Teddy's commitment papers from Dr. Gilchrist and confesses to a confused Elaine about his family's insanity. Determined to do away with his brother, Jonathan ties up and gags Mortimer and is about to kill him when O'Hara returns, having received complaints about Teddy's noisy bugle calls. Seeing Mortimer bound and gagged, O'Hara proceeds to recite the action of his play, while Jonathan lies behind him, unconscious from an accidental blow by Dr. Einstein. When Jonathan revives, he assumes that O'Hara and his partner Brophy are after him and unwittingly reveals that he is a fugitive. Eventually O'Hara's superior, Lieutenant Rooney, shows up and listens in disbelief as Jonathan and his aunts tell him matter-of-factly about the thirteen dead men in the cellar. After Jonathan is arrested and Dr. Einstein slips away, Mr. Witherspoon of the Happy Dale Sanitarium comes for Teddy, only to hear that Martha and Abby, who are anxious to stay near their brother, want to go, too. Just before parting, Abby reveals to Mortimer that he is not really their next of kin but is the adopted son of a sea cook. Overjoyed, Mortimer rushes to tell Elaine, who has since discovered Mr. Spenalzo and Mr. Hoskins for herself, the good news about his parentage and to resume at last their honeymoon plans.