Whitney 'Cam' Cameron [Joseph Cotten] arrives at a hospital and learns that Polly, the stepdaughter of his widowed sister-in-law Lynne, is suffering violent convulsions from an unknown illness. Polly’s condition wavers—she seems to recover, then relapses and dies. The doctors, initially suspecting tetany, are unable to determine the exact cause of Polly’s death.
Maggie [Catherine McLeod] (Maggie Sargent), the amateur sleuth and wife of Cam's lawyer, notices striking similarities between Polly's symptoms and cases of strychnine poisoning she has read about. Cam confronts a troubling realization: his late brother died under remarkably similar circumstances. He hesitates to suspect Lynne Cameron, with whom he has begun to develop romantic feelings. Gary Merrill as Fred Sargent, Cam's lawyer, reveals that the will of Cam's brother leaves the fortune to Lynne only if both of his children die, giving her a plausible motive to kill Polly and the brother. Cam's immediate concern becomes protecting his young nephew, Doug Cameron, Freddy Ridgeway.
Cam arranges an exhumation of Polly’s body for chemical testing and discovers enough strychnine to kill four people, confirming his worst fears. The police ultimately charge Lynne with murder, but a judge dismisses the case for lack of solid evidence. Lynne announces she will take Doug to Europe for a year, and Cam is determined to intervene without revealing his suspicions about Lynne.
He visits a store to learn about strychnine and other poisons and discovers a particularly potent insecticide containing strychnine, marked by a distinctive “W.” He buys arsenic to prepare a final, desperate plan to save Doug. In the meantime, Cam asks for help in gathering clues and continues to weigh his options as the ship sails. The pills in Lynne’s possession include several marked with the “W,” intermixed with aspirin, which convinces Cam that Lynne may have murdered Polly and his brother.
On the Europe-bound ocean liner, Cam pretends to rekindle romance with Lynne to prevent her from leaving, secretly carrying the arsenic nonetheless. He covertly searches Lynne’s belongings and finds the W-mark pills. He hatches a plan to test his theories: in Lynne’s stateroom, he dissolves one W tablet into her drink and warns that the drink could be poisoned—unless she seeks medical help immediately. Lynne acts nervously but denies any wrongdoing, insisting the pills are merely aspirin. The ship’s detective, who had been hidden nearby, steps in, and the time Cam expects the poison to take effect passes. Lynne, furious and indignant, orders the men out of her room. Cam is overwhelmed with guilt for ever having contemplated killing her and feels relief that his earlier plan was aborted.
Shortly after, Cam is summoned to the ship’s doctor and learns that Lynne had rushed to the doctor, dying from a strychnine poisoning just after Cam and the detective had left her stateroom. The doctor is able to save her life with no time to spare. In the aftermath, a court sentences Lynne to life in prison for the murders of Polly and her husband, bringing a tempestuous case to a close.
