Belated honeymooners Polly (Jean Peters) and Ray Cutler (Casey Adams) arrive at their Niagara Falls cottage only to find that Rose (Marilyn Monroe) and George Loomis (Joseph Cotten) have not yet checked out. Though the Cutlers temporarily take another cabin, the lives of the two couples are bound together for the next two days.
Polly discovers that Rose is having an affair and that George, though emotionally unstable, has good reason for his jealous rage. George accurately suspects that Rose openly flaunts her sexuality to make him act crazy in front of witnesses.
This is part of Rose's plan: her lover Patrick (Richard Allan) will kill George and make it look like suicide or a disappearance. Instead, George kills Patrick, and he returns to kill Rose, but finds Polly instead. As she had been sympathetic to him, he asks her not to tell anyone that he is alive so he can simply disappear.
But, realizing that he wants to kill Rose, Polly informs the police. What follows is escalating terror, with George stalking Rose, Rose desperately trying to leave town, the police searching for both of them, and finally George and Polly adrift in a boat heading for the precipice.
In Henry Hathaway's Technicolor film noir, Niagara Falls serves as an apt metaphor for the destructive power of out-of-control carnal and murderous obsessions.