David I. Starrling, a loving husband and devoted father, returns to his New York City apartment, following a brief business trip, only to discover that his wife Celia has been having an affair with Crane Weymouth, the fiancé of Celia's sister, Dell Faring. David shields his six-year-old daughter Susan from his discovery, but reveals his knowledge to Dell, Crane and Celia later that evening during a dinner party. At the end of the evening, after Dell and Crane have left, David angrily confronts Celia, and as he approaches her brandishing his souvenir handgun, she panics and knocks him unconscious with a hand-held mirror. Moments later, Dell returns to the apartment and finds her sister crying in her bedroom, certain that she has inadvertently killed her husband. After reassuring Celia that David is merely unconscious, Dell places David's gun in her coat pocket and accuses Celia of stealing her fiancé. As her anger rises, Dell reaches into her coat pocket and fires the gun at Celia, killing her instantly. Just then, Susan enters the bedroom, sees her parents lying on the floor and screams. David eventually regains consciousness and is later tried for the murder of his wife. The jury finds David guilty of first degree murder, and the judge sentences him to death. Because he is unable to remember what happened after he was knocked unconscious by his wife, David accepts the verdict and resigns himself to spending the rest of his life on death row. Dell, who watched the trial in silence, continues to suppress the truth about the murder, even when she becomes tormented by feelings of guilt. Susan, who has developed psychological problems after seeing her mother dead, is placed under the care of psychiatrists Dr. Hodge and Dr. Caroline Canford at Children's Hospital. After studying Susan's play through a one-way mirror, Caroline begins to suspect that there may have been a third person in the bedroom when Susan entered it. Dell eventually learns about the details of Caroline's examination of Susan and, fearing that the truth may surface, tries to silence the girl by poisoning her chocolate milk. Her plan fails, however, when Susan accidentally spills the drink. After gaining legal custody of Susan, Dell makes plans to take her to her vacation home in Connecticut. Caroline, however, objects to the trip because she believes that Susan is close to identifying the mysterious person who was in the bedroom at the time of the murder. Because she is unable to prove her suspicions, Caroline is forced comply with Dell's wishes, and she and Dr. Hodge drive Susan to Dell's country home. Just as Caroline and Dr. Hodge are about to leave, Dell turns on a porch light, which casts her shadow onto the side of the house. Susan screams when she recognizes the shadow as being the same one she saw in her parents' bedroom on the night of the murder. Dell then makes a complete confession, after which David is exonerated and released from prison.