When enraged businessman David Starrling (Zachary Scott) confronts his cheating wife, Celia (Kristine Miller), he is rendered unconscious by a mysterious figure and wakes up to find Celia murdered. David and Celia's young daughter, Susan (Gigi Perreau), witnessed her mother's death but saw only a shadow and is unaware that the killer is Dell (Ann Sothern), Celia's scorned sister. Dell -- terrified of discovery -- must now find a way to do away with her own niece before she learns the truth.