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When his friend Robert Grell (Denton Vane) is found murdered, detective Heldon Foyle (Earle Williams) has quite a case on his hands. The dagger that killed him has the fingerprints of Helen Meredith (Miriam Miles), Foyle's sweetheart and sister of Grell's fiancée, Eileen (Jean Dunbar). But Grell's servant Ivan (Bernard Siegal) is also a suspect. It takes quite a bit of work on Foyle's part to uncover the truth -- Grell had an evil twin brother, Harry Goldenberg (also played by Vane), and he is the one who was found murdered. The killer is Goldenberg's wife, Lola (Mabel Trunnelle), who was trying to protect Grell from her husband. Grell has been in hiding, and he comes out after Lola has confessed. He and Eileen are reunited, while Foyle marries Helen. This sharp murder mystery kept everyone guessing until the end.