When businessman Ray Cutler wins a sales campaign contest at work, he is awarded a trip to Niagara Falls, and takes his wife Polly on a belated honeymoon. Upon their arrival, however, they learn that their intended cabin has not yet been vacated by the current residents, George and Rose Loomis. The beautiful, young Rose tells Mr. Qua, the motel manager, and the Cutlers that her husband has been ill and begs them not to wake him, so the Cutlers accept a different cabin. Rose also reveals that George had spent time in a military psychiatric hospital, then states that she is going grocery shopping. A short time later, however, Polly spots Rose engaged in an intimate embrace with a man at Horseshoe Falls. That night at the motel, Rose asks some dancing teenagers to play a record of her favorite song, "Kiss," and an enraged George storms out of their cabin and smashes the record. Rose tells the Cutlers that George's temper frightens her, and Polly offers to tend to George's hand, which has obviously been cut by the record. In the cabin, George tells Polly that Rose is a tramp but admits that he loves her anyway. Ray joins them, and George reveals that he lost his farm because Rose was bored with country life, and then volunteered for duty in Korea to prove to her that he was young enough to serve. He was sent home with battle fatigue, however, and since then has been tormented by the idea that Rose is having an affair. Nonplussed by George's erratic behavior, Ray and Polly leave, while in the office, Rose calls her lover, Ted Patrick, and tells him that they must put their plan to kill George into action in the morning. Ted agrees, and the next day, a suspicious George follows Rose when she states that she is going to buy bus tickets for their trip home. Instead, Rose goes to the Horseshoe Falls gift shop and there eludes George, who takes the elevator to the attraction. After leaving Rose a card informing her that if all goes well, the bell carillon will play "Kiss," Ted follows George. Later in the day, Rose worriedly asks Qua, Polly and Ray if they have seen George. On Rose's behalf, the Cutlers file a missing persons report, and in the evening, when the attendant at the Horseshoe Falls discovers that one rain slicker is missing, and one pair of shoes has not been claimed, he alerts Inspector James Starkey. Starkey asks Rose and the Cutlers to accompany him, and Rose identifies the shoes as George's. As Rose walks home, she hears the bell tower playing "Kiss," and smiles. Soon after, the police discover a body along the shore and ask Rose to identify it. When Starkey raises the sheet covering the corpse, however, Rose faints and must be hospitalized and sedated for hysteria. At the cabins, Polly meets Jess Kettering, the vice-president of Ray's firm, and his wife, and they offer to take the Cutlers sightseeing. Polly promises to call them when Ray returns from town, then is told by Qua that their luggage has been moved to the Loomis' cabin. There, Polly lies down for a nap, and is startled when George, believing her to be Rose, menaces her with a knife. Polly's screams drive George away and attract Ray, who dismisses her account as a nightmare. Unknown to the Cutlers, the corpse seen by Rose was Ted's, and George is now determined to kill Rose. Later that afternoon, the Cutlers go sightseeing with the Ketterings, and George singles out Polly and tells her of Rose and Ted's plot to kill him and make his death look like suicide. Believing that George acted in self-defense when he killed Ted, Polly advises him to turn himself in, but George states that he wants to disappear and start a new life. After he leaves Polly, however, George requests that the bell tower play "Kiss," and when Rose hears it in the hospital, she is so terrified that she flees, despite her drugged state. Starkey informs the Cutlers of Rose's escape, and Polly tells him that George must be trying to kill her. Rose attempts to reach the American side of the falls, but George follows her, and in desperation, she climbs to the top of the bell tower. There, George strangles her and upon discovering that he has been locked in the building for the night, returns to Rose's body and tells her that he truly loved her. The following morning, while the police hunt for George, the Ketterings take the Cutlers fishing. The group stops to buy gas and provisions, and Polly is the first to return to the boat. George has snuck aboard, and when she tries to stop him from stealing the boat, he accidentally knocks her unconscious. George is seen as he departs, and Starkey calls American river patrol officer Morris to track the stolen boat. When Polly revives, George confesses that he killed Rose, but is interrupted as the boat runs out of gas and gets caught in the rapids leading to the falls. Morris' ship is forced to turn back, and the horrified Ray prays that George will scuttle the boat before it reaches the falls. George does attempt to sink the boat, but cannot stop it in time. Fortunately, the boat passes a huge rock in the middle of the water, and George succeeds in pushing Polly to safety just before the boat plunges over the falls. Polly is rescued by a U.S. Coast Guard helicopter, and later, as Ray embraces his shaken wife, Starkey wishes them well.