Whitney "Cam" Cameron arrives at a hospital to be with his widowed sister-in-law Lynne, whose stepdaughter Polly has died under mysterious circumstances. A doctor cannot determine the cause of the child's death.
Cam has great affection for his young nephew Doug. He begins to fear for the boy's life when Maggie Sargent (Catherine McLeod), the wife of his lawyer, Fred, mentions that the dead girl's symptoms sound suspiciously as if she had been poisoned.
Fred reveals that the will of Cam's brother, who also died from unspecified causes, put all his money into a trust for the boy. Lynne would inherit it all if anything happened to Doug.
Police, prodded by Cam, exhume the girl's body. Poison is found and Lynne is brought to court, but a judge dismisses the charges for a lack of evidence against her.
A desperate Cam cannot think of any way to keep Doug safe, particularly once Lynne decides to take the boy away to Europe for at least a year. Cam surprises them by turning up on the ocean voyage. He begins romancing Lynne, all the while plotting to poison her.
He slips a tablet from her belongings into a cocktail. Lynne goes to great lengths to castigate Cam for his suspicions and demonstrate that the tablet contained nothing but aspirin. Cam leaves her stateroom, but a few minutes later Lynne's life is saved by the ship's doctor .....