Marilyn Monroe and Joseph Cotten are an ill-suited couple staying in the cottage next to happy newlyweds Casey Adams and Jean Peters near Niagara Falls. While sightseeing, the newlyweds see Monroe kissing her young lover (Richard Allen) and hear them plot to kill her husband, making it look like suicide. When the latter goes missing, Monroe is asked to identify a body and discovers it's her lover. Her husband's vengeance threatens them all...