When an ex-Hungarian turns up mysteriously after having been missing for a whole year, is taken ill and then dies from an overdose of drugs, Steve Gardiner sets out to retrace the man's footsteps - and discovers a sinister clinic run by an ex-SS doctor.
When Gardiner organises a British tour for a South American dance company and discovers a plot to discredit their patron, he intervenes on the British Government. But South, a hired killer, stalks his every move.
When Professor Preece is sent to prison for passing secrets to a foreign agent but escapes to settle accounts with the real traitor (his son), a chance meeting with Judy alerts Steve Gardiner to Preece's plans.
Before Gardiner can arrive at a sleazy hotel to keep a rendezvous with a man claiming to have important news, South kills the informer. Steve discovers the man's body and a reel of film - which sets him on the trail of a powerful, evil villain.
A young boy 'adopts' Judy and declares that he will stay with her and Steve, and then produces a £100 note to pay the rent. This odd event leads Steve to uncover an uncover an assassination plot against a French politician, whom South has been hired to kill.
When Judy has a nightmare during which she is murdered by a mute killer and then meets South, the man from her dreams, in corridor of a train, Steve finds more than he bargained for on his return home - Judy is dead.
Serve is approached by a young girl who asks him to sign a petition on behalf of the man who killed his wife and who is applying for a reprieve. They argue and the incident places both their in danger when Steve is mistaken for an American journalist.
The body of an actor, covered with flowers and found in a car, leads Steve to bait a trap for the killer - South, the mysterious assassin who murdered Judy. Steve unmasks the killer and South is brought to justice and hanged.