After overcoming the fear of marriage, New York theater critic Mortimer Brewster, the bachelor who wrote millions of articles against marriage, married the priest’s daughter Elaine Harper on Halloween. On the way to Niagara Falls for their honeymoon, Mortimer and Elaine stopped for a while in Brooklyn. Elaine's father lives in Brooklyn, and Mortimer's two aunts live next to Harper's house. When Elaine conveyed the marriage news to her father, Mortimer went to visit her aunt Abby and Martha. When Mortimer was looking for notes on his writings in his aunt's living room, he found a dead body in a box by the window. Mortimer thought it was his crazy uncle-he thought he was Teddy Roosevelt-doing a good thing. However, to his shock, Abby and Martha calmly confessed to the murder of Mr. Hoskins. They confessed that they not only killed Mr. Hoskins, but also killed 11 other people, all of whom are now buried in the cellar.
The murder story of the old woman started when an elderly visitor died of a heart attack in the living room, which inspired them. Mortimer tried to point out that they were wrong to kill. On the contrary, Abby and Martha insisted on using "rental houses" to lure lonely old people into their homes and provide them with poisoned elderberry wine, which is regarded as a charitable service. When Mortimer frantically managed to get his known mad uncle into a nursing home, lest the aunts be accused in the future. And another old man Gibbs who might rent a house came to this house. After preventing Gibbs from drinking the deadly elderberry wine, Mortimer left to see the judge to get a signed undertaking.
When Mortimer went out, his mentally ill villain brother Jonathan unexpectedly appeared in the house with an alcoholic underworld plastic surgeon Einstein after escaping from an Indiana prison. Unbeknownst to the aunts, Jonathan also brought with him the body of Spanaso that he had murdered. Einstein changed Jonathan's face into the like of Boris Karloff. According to the wishes of his sisters, Teddy buried Hoskins as a yellow fever patient in the cellar in the "lock" of the "Panama Canal" like other victims. He invited Einstein to inspect his newly dug hole in the cellar. Einstein thought that the hole could be used to bury Spanaso. Jonathan forced his aunt to rest early, and then moved Spanaso to the living room and put it in the cellar. However, before Jonathan and Dr. Einstein moved Spanaso to the cellar, Elaine came, and they had to store the body in a box by the window. Jonathan, who is always paranoid, then tried to drag Elaine into the cellar, but was unsuccessful because Mortimer rushed back in time. Mortimer sent Elaine home, and then he found Spinaso's body in a box by the window. Although Mortimer first suspected that this was the work of his aunt, Jonathan hurriedly sat up when his aunt was about to open the box, revealing that he was the culprit. Before Mortimer took action, the new police officer O'Hara came to their house. Mortimer promised to discuss his script with O'Hara later, and finally got rid of the police. Then Mortimer met Dr. Einstein. Einstein accidentally discovered Mr. Hoskins's body while disposing of Spinalzo's body. After Jonathan and his aunt argued over who had the more impressive murder record, Mortimer obtained the second signature needed to send Teddy away from Dr. Gilchrist.
Jonathan was determined to have a break with his brother. He tied Mortimer and wanted to kill him, but at this time O'Hara heard Teddy blowing the horn to deal with it. O'Hara saw Mortimer being tied up but thought he was acting, and continued to recite his play. Jonathan tried to attack O'Hara behind him, but Einstein knocked out Jonathan. When Jonathan woke up, O'Hara’s partner Brophy found that Jonathan was a fugitive. O'Hara's boss, Lieutenant Rooney, didn't believe Jonathan's words that his aunts had thirteen dead in the cellar. Jonathan was arrested, Einstein slipped away, the nursing home came to pick up Teddy, and Martha and Abby, who were eager to stay with their brothers, also wanted to leave. Before breaking up, Abby revealed to Mortimer that he is not the real Brewster family's flesh and blood, but the adopted son. Mortimer was overjoyed and hurriedly told Elaine the good news. Although Elaine found the body at this time, Mortimer resumed his honeymoon plan
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