During an open-mike session at a coffeehouse in San Francisco, Charlie Mackenzie recites a poem he has written about his most recent romantic failure. In the audience is his best friend, police detective Tony Giardino, who chides him about his inability to commit to one woman. Charlie argues that one woman he rejected had ties to the mafia, another smelled like beef vegetable soup, and that his most recent girlfriend stole his cat. Tony doubts these excuses, but Charlie admits he is afraid of marriage and says he has not yet met the right person. The next day, Charlie sees a beautiful woman working at the specialty butcher shop, Meats of the World, where he purchases haggis for his Scottish parents, Stuart and May. That night before dinner, May expresses regret that Charlie broke up with his most recent girlfriend, then shows him an article in the Weekly World News about a serial killer, dubbed “Mrs. X-Honeymoon Killer.” According to the article, the axe murderer, who has not been apprehended, killed three husbands: a lounge singer in Atlantic City who specializes in the song “Only You,” a Russian martial artist in Miami and a plumber in Dallas named Ralph Elliot. Convinced that the story is contrived tabloid fodder, Charlie suggests that May read a newspaper that reports facts. Charlie is unable to get the beautiful butcher, Harriet, off his mind and returns to her shop. Finding the shop crowded, he offers to help her serve the large number of customers. After a hectic day working together, Charlie and Harriet spend the evening eating hot dogs and walking around the city. When Russian sailors walk past them, Harriet speaks to them in their language. Later that night, Charlie accompanies Harriet to her apartment, which she has decorated with a giant Atlantic City poster and a display of martial arts items. She explains that she used to live in Atlantic City and that she had a friend who was a martial arts expert. Invited to spend the night, Charlie is later awakened when Harriet screams the name, Ralph, in her sleep. In the morning, Charlie enters the shower and inadvertently walks in on Rose, Harriet’s eccentric sister, who says she lives there and warns him to “just be careful.” Although he finds Rose’s behavior odd and wonders about Ralph, Charlie later tells Tony that he is smitten with Harriet and wants the relationship to work. When Charlie takes Harriet to his parents’ house for dinner, he reads the Weekly World News article about Mrs. X that has been left in the bathroom, and notices similarities between Harriet’s background and that of the axe murderer. Shaken, Charlie questions Harriet about her ties to Atlantic City and her Russian martial artist “friend,” but Harriet brushes aside his questions. Charlie asks Tony to investigate the case, but his friend dismisses Charlie’s fears and presumes they are a manifestation of his commitment-phobia. That night, Charlie sees a news special about family members who intentionally poison loved ones, then suspects Harriet of trying to kill him when she offers him a “health shake.” When she talks about trust and vulnerability in relationships, he panics and makes her feel uneasy. The next day, Charlie places an announcement for his parents’ thirtieth wedding anniversary in the newspaper. At the newspaper office, he overhears two obituary writers discuss the death of plumber Ralph Elliot. In a moment of terror, Charlie recognizes the name as one of Mrs. X’s murdered husbands and asks the writers if they know anything about the man’s wife. They do not, which increases Charlie’s suspicion that Harriet is the serial killer. He asks her to meet him in a public park, where, claiming he has a fear of rejection, he breaks up with her. Although she does not believe his reason for severing the relationship, she says that at least he broke up with her early in the relationship. Days later, Tony informs Charlie that a woman confessed to the murder of Ralph Elliot. Believing Harriet is innocent, Charlie rushes to her apartment to profess his love. After some pleading, and serenading her with poetry and music, Charlie convinces Harriet to take him back. She introduces him to her friend, Ralph, who, Charlie is surprised to learn, is a woman. Harriet then accompanies Charlie to hi parents’ anniversary party, where Charlie, swept up by his mother and father’s continuing love affair, proposes. Although Harriet hesitates at first, she accepts and they later wed in a Scottish ceremony. At the reception, where a drunken Stuart sings vulgar songs accompanied by bagpipe, Harriet serenades Charlie with a bilingual rendition of “Only You.” The next day, Charlie and Harriet head off to their honeymoon at the secluded hotel, Poets Corner, as a massive storm approaches. Meanwhile, in town, Tony learns that the woman who confessed to killing Ralph Elliot also claimed to kill Abraham Lincoln and Julius Caesar. Unable to call Charlie due to the storm, Tony charters a plane to deliver the information in person. After the plane lands, Tony calls the hotel from a phone box and tells Charlie he believes Harriet is Mrs. X, but before he can provide more information, the phone line goes dead. Feeling trapped, the panicked Charlie tries to distance himself from Harriet, who becomes uneasy. Charlie invites the bellboy to their room for a drink, but is declined. When Harriet begins to confess that she has been married before, Charlie cuts her off and, without hearing her story, locks her in the closet. He then finds a forged note to Harriet, purporting to be from him and stating that he is leaving her. Suddenly Rose emerges from a shadowy corner of the room with an axe. She tells Charlie that it is time for her to get rid of him, as she did all the other husbands who took Harriet away from her. Charlie escapes out the window and onto the roof, but Rose follows, swinging the axe. Tony arrives at the hotel and, after releasing Harriet from the closet, puts her in handcuffs. Harriet manages to convince Tony of her innocence and directs him toward Charlie and Rose, who are struggling on the rooftop. Charlie fends off Rose’s attacks, but when she tumbles over the roof’s edge, he grabs her hand to keep her from falling to her death, and Tony pulls her to safety before arresting her. Some time later, at a coffeehouse performance, Charlie recites a poem about Harriet, who until recently believed her three husbands had abandoned her. With Rose in jail and Harriet finally aware of her sister’s murderous past, Charlie and Harriet are free to start their life together as a married couple in San Francisco.