Two couples are on second honeymoons, and the ordinary couple is drawn into a murderous plot involving the other estranged couple.
George Loomis (Joseph Cotton) and Rose Loomis (Marilyn Monroe) are staying at Niagara Falls. Polly (Jean Peters) and Ray Cutler (Max Showalter) arrive on a delayed honeymoon and have reserved the same cabin. Rose says her husband is not well, and the Cutlers take another cabin. The Cutlers visit the falls on a tour, and Polly sees Rose kissing another man.
In their cabin George finds her ticket to the falls. At a dance Rose is in a sexy dress and puts on her favorite record. She sits by the Cutlers and sings “Kiss Me.” George comes out of the cabin, breaks the record, and goes back in the cabin. Polly goes in to treat his cut hand, and he says Rose is a tramp. Ray comes in and meets George, who tells how he met Rose and his life got fouled up. He kicks over a table, and the Cutlers leave. Rose calls Patrick (Richard Allan) at a boarding-house and tells him their plan will work because George showed himself to be unhinged.
In the morning George promises to buy Rose a new dress. She says she is going to the bus station to buy tickets, and he is suspicious. George looks around the bus terminal and follows Rose. She and Patrick nod at each other, and she gets a message to meet her he wrote on a postcard. Ray is taking a photo of Polly in a swim-suit when Rose asks where George is.
Inspector Starkey (Denis O’Dea) learns that the Cutlers reported George missing. He meets with them and Rose at the falls. A rain-suit is missing, and Rose identifies George’s shoes. The Cutlers are to take Rose home, but she hears the bells and decides to walk. A body is found, and Rose meets Starkey at the morgue. When she sees the body, she faints.
In a hospital a doctor gives Rose a sedative. Mr. Kettering (Don Wilson) and his wife (Lurene Tuttle) arrive and are glad to see Polly, who learns her things were moved to cabin B. She closes the blinds and lays down. George comes in and gets a knife. When Polly sees him and screams, George leaves. Ray breaks in and tries to calm down Polly. She says she saw George, but Ray thinks he is dead and does not believe her. She says the Ketterings are there. Polly goes to a phone and calls Starkey, leaving a message. George overhears her say his name.
The Cutlers and Ketterings go on the tour of the falls. Polly goes off on her own and sees George in a rain-suit. She waves to the others and flees from George, who grabs her. She says he killed that man; but he says it was self-defense. George says Rose tried to kill him. Polly goes back to the others. In the hospital bed Rose hears the bells playing her song.
The Ketterings and Cutlers plan to go fishing at 5 a.m. Polly finds Starkey in the cabin, but she says she only had a silly question. Starkey says that Rose escaped from the hospital. Ray comes in, and Polly says Loomis is alive and trying to kill his wife. Ray asks Starkey to leave them alone.
Rose gets a refund on one of her tickets to Chicago. She hears that the police are checking everyone’s identification, and she leaves. She talks to a cab driver and learns of a road-block but that she can walk across the border. She sees George waiting and avoids him. She tries to call the police, but she sees George and flees up stairs. He catches up to her in the bell tower and strangles her to death. He is locked in the building and finds her lipstick. He tells her body he would have helped her. A janitor opens the building, and George goes out.
Don Kettering drives a fishing boat, and the two couples go for supplies. A boatman (Will Wright) tells two motorcycle cops he saw Loomis trying to get a boat. George overheard this and uses a stick to set off a car horn. The boatman comes over, and George goes on the boat but needs the key. Polly comes back, and George tells her to get off. He knocks her down and hot-wires the boat, taking off. The boatman calls the police. George asks Polly if she is all right. She advises him to tell the police; but he says it is too late because he killed Rose.
Starkey calls the river patrol, which pursues George’s boat. George tries to scuttle the boat. A helicopter takes off. George pushes Polly off to a rock in the river before he goes over the falls in the boat. Polly waves to the helicopter, which rescues her with a suspended chair. On land Polly thanks Starkey.
This suspenseful mystery contrasts the normality of the Cutlers and Ketterings to a neurotic couple with a sexy woman involved in adultery and murder. The irony is that she drives her targeted husband to insanity and murder and thus her own undoing.