Ray, portrayed by Casey Adams, and his wife Polly Cutler, played by Jean Peters, are attempting to enjoy a delayed honeymoon at the breathtaking Niagara Falls. Upon arrival at their reserved cabin at the Rainbow Cabins on the Canadian side, they discover it is occupied by George, depicted by Joseph Cotten, and his wife Rose, portrayed by Marilyn Monroe. Rose argues that George has been unwell and persuades Polly and the cabin manager, Mr. Qua, to allow them to stay, prompting the Cutlers to accept a less desirable cabin.
Ray is on a business trip after winning a prize for his creative sales campaign at his company, which specializes in shredded wheat crisps. Meanwhile, George works at the Toledo plant and wants to meet with the head of the Niagara factory, Mr. Kettering, although he learns that Kettering is out of town.
As they get to know each other, it becomes evident that George and Rose's marriage is strained. Rose's youth and beauty contrast sharply with George's jealousy and irritability; it’s suggested that he may have recently left an Army mental hospital. The couple generally keeps to themselves, and Rose often appears indifferent to George.
On a tour of the falls the next day, Polly witnesses Rose engaging in a passionate kiss with a man named Patrick, played by Richard Allan. In an unusual stipulation, visitors are required to wear galoshes, leading to an amusing encounter. Back at their cabins, George discovers a Niagara tourist tunnels ticket in Rose's coat, indicating her escapades without him.
That night, the Cutlers bear witness to George's angry outburst when he suspects Rose's disinterest in their marriage. After a brief gathering, George's turmoil manifests as he storms out, shattering a record in a fit of rage related to a song he believes contains hidden meaning for Rose. Concerned about George's state, Polly visits him to provide first aid for his hand, and he opens up about his troubled past as a sheep rancher whose fortunes dwindled after marrying Rose, who he had met while she was still a barmaid.
Unbeknownst to George, Rose is plotting his murder, instructing Patrick to prepare for the following day. She manipulates George into following her to a dark tourist tunnel, where Patrick awaits, intending to kill him. As part of her scheme, she tells George she is going to buy bus tickets for Chicago, knowing he would follow.
However, in a twist of fate, George ends up killing Patrick in self-defense and cleverly disguises his actions by taking Patrick's shoes. When Rose alerts the police to George's strange disappearance, Inspector Starkey arrives to investigate.
Despite Rose's assumptions about George's fate, George emerges as the character who has triumphed over Patrick, allowing the police to mistakenly believe he is the victim. When they retrieve a body and bring Rose to identify it, she is devastated to find that it’s Patrick instead of her husband, ultimately leading to her hospitalization.
As events unfold, Mr. Kettering pays the cabins a visit to meet Ray, prompting the motel manager to relocate the Cutlers to the Loomises' cabin. George is now on a vengeful path, seeking retribution against Rose but stumbles upon Polly instead. Their encounter leads to a complicated emotional exchange, and Polly soon relays her suspicions about George's survival to the police.
Later, as chaos ensues, George confronts Rose, leading to a dramatic confrontation within the Carillon bell tower, where he tragically strangles her amidst a silent backdrop. Meanwhile, the Cutlers find themselves fishing with friends on a section of the River Niagara, where George seizes the opportunity to kidnap Polly, leading to a terrorist chase that ends with George sacrificing himself as he plunges over the Falls. The story leaves a haunting image of Rose’s lifeless body in a striking red dress at the base of the cascading waters, while Polly is ultimately rescued, highlighting the tangled web of love, betrayal, and tragedy at Niagara Falls.
