The young Dr. Cassilis, assistant to that imminent pathologist and specialists on poisons, Sir Frank Tarleton, has a secret to hide and a young lady in jeopardy to protect. All of which he must hide from his intelligent and alert employer as they investigate their latest case.
Early one morning, Dr. Cassilis takes a call from the authorities asking for Tarleton to come to The Domino club, a shady nightclub where the patrons carouse while their identities are hidden by masks and costumes. A man has been murdered, and the authorities are concerned it was an attempt on a visiting prince attending the club and want the murder hushed quickly. But, as Tarleton and his young assistant Dr. Cassilis investigate, it quickly becomes clear that the victim, a 'doctor' has been using the club as a front for blackmail.