London police constables are being killed by a man calling himself "Mr. X" (Leonard Mudie). By chance, one of the murders occurs around the same time and place as a diamond robbery, leading Police Commissioner Sir Herbert Frensham (Henry Stephenson) to suspect the same man is responsible for both, much to the annoyance of the thief, Nicholas "Nick" Revel (Robert Montgomery), and his confederates, taxi driver Joseph "Joe" Palmer (Forrester Harvey) and insurance clerk Hutchinson (Ivan F. Simpson).