Two orphans, Polly and Doug, live with their stepmother Lynne; Polly collapses with the same mystery symptoms that killed her father. The kids' visiting uncle, Whitney Sargent, is warned that the symptoms match strychnine poisoning, but that poisoners are seldom detected and rarely convicted. Sure enough, no case can be made against the obvious suspect; so what can Whitney do to save the next victim?

Businessman Whitney "Cam" Cameron rushes to the hospital when his niece Polly is felled by a mysterious illness. Dr. Stevenson is baffled by Polly's painful seizures and her cries of "don't touch my feet," but reassures Lynn Cameron, Polly's stepmother and the widow of Cam's late brother Bill, that Polly will recover. After spending the evening by Polly's bedside, Cam and Lynn go home, where they are greeted by Lynn's young stepson Doug, who remarks that Polly's illness is strikingly similar to that suffered by his father before his death. Although it appears that Polly will survive, a sudden relapse kills her the next night, and the grief-stricken Cam turns to his friends, Fred and Maggie Sargent, for comfort. Maggie, a reporter, comments that Polly's symptoms sound suspiciously like poison, and explains that the majority of poisoning murders go undetected. Cam and Fred dismiss Maggie's idea as outrageous, as does Dr. Stevenson, although Maggie is intrigued when Cam admits that Lynn refused to allow an autopsy of Polly's body. Cam spends the following week with Doug and Lynn, and is impressed by Lynn's gentle treatment of her stepson. On the night before Cam is to leave, however, Fred asks him to stay, explaining that when he helped Bill to write his will, Bill stipulated that Lynn could not inherit his fortune unless both of the children died first. Cam is even more shocked when Fred comments that if Lynn is guilty of murdering Polly, she would have demanded that her corpse be cremated, which she did. Fortunately, Cam insisted that Polly be buried, and Fred arranges for her body to be exhumed. The autopsy confirms that Polly died of strychnine poisoning, and Assistant District Attorney Hal Cole and homicide captain Pringle agree to investigate. Lynn readily submits to questioning, as do the Cameron servants, and the police are frustrated by the lack of evidence. Cam, who still refuses to believe that Lynn could be guilty, is horrified when his own inquiries reveal that she purchased the last prescription of calcium capsules administered to Polly, which had to contain the fatal dose of strychnine. Cole and Pringle question Lynn again, and although she maintains her innocence, their certainty that she is guilty finally convinces Cam. Knowing that Lynn intends to take Doug to Europe, Cam fears that she will poison him, too, and Cole reluctantly agrees to arrest Lynn, even though their evidence is circumstantial. During the probable cause hearing, the case against Lynn is dismissed for lack of evidence, and Lynn, who does not know that Cam was instrumental in her arrest, is greatly relieved. Hoping to obtain custody of Doug, Cam applies to Judge James J. Adams, but Adams tells him that because the case against Lynn was dismissed, there is nothing he can do. Desperate to protect Doug, Cam decides to take the same ocean liner on which Lynn is departing for Europe, but before he leaves, he visits a nursery to buy some arsenic insecticide. While there, Cam also sees some strychnine tablets, used to kill rodents, which look just like aspirin except for a distinctive "W" imprint. Later, although Doug is delighted to see Cam aboard the ship, Lynn responds coolly. Cam schemes to keep watch over Doug by pretending to romance Lynn and the five-day voyage passes quickly. Cam, who has hidden the insecticide in a bottle of aftershave, is wracked by doubts over Lynn's guilt, but, knowing that there is no other way to save Doug, decides to poison her cocktail one night. Cam cannot bring himself to do it, however, and spends one last romantic evening with Lynn. When Cam briefly returns to Lynn's cabin to retrieve her coat, he sees her box of toiletries, and wonders if she could have hidden poison in the bottles, as he did. When he searches through a bottle of aspirin, Cam finds three of the "W" strychnine tablets and resolves to kill Lynn. Insisting that they return to her cabin for a nightcap, Cam puts one of the tablets into Lynn's drink, then announces what he has done. Frank Connelly, the ship's detective, had been alerted by Cam and is there as a witness when Lynn declares that the tablet was aspirin and therefore harmless. As ten minutes pass, Cam pleads with Lynn to confess and see the doctor while there is still time to save herself, but she angrily castigates him for his deviousness. Fed up, Connelly makes Cam leave, but soon after, while Cam stands on the deck reproaching himself, he receives word that Lynn is in the doctor's office. There, Cam learns that she phoned for the doctor immediately after he and Connelly left, and that she was barely saved from dying of strychnine poisoning. Later, Lynn is convicted of killing Polly, and Cam and Doug depart for a new life together.