After a robbery in his london shop, antique dealer John Mannering a.k.a. The Baron, is persuaded by Templeton-Green to work for Diplomatic Intelligence. He travels behind the Iron Curtin and meets his contact Cordelia. They encounter a corrupt Party member who attempts to frame them for the murder of his lover, the London jewel thief.

Mannering attends the funeral of Jim Carey, a former army colleague. After receiving a message that Carey is still alive, Mannering agrees to go undercover for Templeton-Green. An elaborate plan to steal a $ 1 m jewel is bad enough but becomes worse when Cordelia is used as a hostage to ensure that the Baron cooperates with Carey.

The Baron does a deal with a scheming insurance agent to recover a £ 100,000 Aztec mask from an ex-con. However the former crook's gang leader has a different idea and will even hurt his one-time friend's daughter to get the mask.

When Mannering gets a message to buy The Four Horsemen, he travels to Khakania, a country gripped in civil war. The Horseman are a famous statue worth over $1m. The money from the statue will help fund the rebel cause. The Baron tries to flee the country with the rebel leader's daughter and statue but is hounded by the secret police who are always one step ahead of him.

An undercover British agent is caught in an Eastern Bloc country and names Mannering as his contact. The Baron and Cordelia are in the country to pass over some money. They are warner not to go to the rendezvous by Bradfield as it is a trap. Cordelia is caught and held by the police. Mannering has a plan to kidnap the head of the police but there is a double agent in the Bradfield house.

Mannering is invited by Sir Frederick Alton to his country home. When he enters the house he hears a woman crying for help. As he goes to help he's knocked unconscious. He wakes up to discover that he has a twin double. Mannering manages to change places with the double and his captors kill the man they belive to be the Baron. They then let him in on their plans to steal the Crown Jewels only for Cordelia to turn up at the wrong moment.

The Baron has to convince Cordelia who he is, but he still has to play the part for his captors. He manages to get rid of Revell and the gang head to London. However, one of the gang is the best friend of the part Mannering is playing. After he's discovered, the Baron really has to steal the Crown Jewels.

David Marlowe is kidnapped from Mannering's shop. For his release he must sell a false Renoir to Sir Robert Ellacott. Mannering calls on Templeton-Green for help. The Baron follows his orders and sells the picture but whilst visiting he meets Sir Robert's nephew Roddy and becomes suspicious. When Marlowe is stabbed by Roddy as he attempts to escape, the other gang members become nervous. Mannering has to meet Roddy with the ransom money to ensure that Marlowe isn't killed.

The Baron and Cordelia are in Switzerland when they bump into Peter Franklin, an old friend of Cordelia. Franklin, who is a CIA agent, has stolen the latest strain in a deadly virus from the laboratory he was working in. When Franklin is killed, Mannering is in a race with Sorenson, the scientist who developed the virus, and Holmes, an unscrupulous character, to discover the whereabouts of the deadly bacteria.

Mannering is asked by an eccentric millionaire to witness him purchase The Legions of Ammrak, a golden necklace that contains seven perfectly matched black pearls. The Baron identifies that they are genuine but David Marlowe notices that the seller, King Ibrahim, is wearing the wrong school tie. When Mannering realises that he's being duped, he has to recover both the money and the necklace.

The sale of a 5000 year old Samurai sword gives the Baron trouble. When the seller, Asano, bumps into an old enemy, Norman Sterling, at Mannering's apartment, Sterling promises to avenge the treatment of him and his brother in a Japanese POW camp. After Asano is murdered, the Baron has to protect Sterling from Asano's loyal servant is out for revenge.

Mannering stops up to pick up a petrified girl in a country lane, as they drive away they are shot at and crash off the road. When he comes to, he keeps having flashbacks that feature weird objects but the police simply think that he is suffering from concussion. As he tries to retrace his footsteps, he is watched by a man with a gun who is determined not to let him near the house in which the girl is held hostage.

Louisa Trenton, an old friend of Mannering is in trouble. When she sells the Baron some rare family miniatures he offers his help but she declines. Later when she calls for his help he rushes to meet her in a London club only to find that she has been murdered. When he investigates he discovers that she was being blackmailed by her young lover. He is only the fall guy but as Mannering tries to find out who is the boss, more people are murdered.

Mannering gets a tip-off that there will be a robbery at the Lynstead Collection. The police and the Baron lie in wait but one of the policemen is killed by the gang leader, Greg Wilde. Wilde is arrested for murder but his crooked solicitor tells him that all of the witnesses will be disposed of. Mannering escapes an attempt on his life but Wilde escapes from police custody and is out to get the Baron.

The Baron and Cordelia are at the Macao docks checking a shipment of antiques. A missing crate leads to Cordelia witnessing a murder on baord a cargo ship. When she is held prisoner Mannering has to stow away on the ship. They meet an undercover CIA agent working as one of the crew who tells them about the ship's mysterious cargo and its secret destination. The CIA agent is killed and Mannering forced out into the open.

With the ship arriving at the island to deliver its cargo, Mannering learns of the plot to steal an American space capsule. Via the ship's radio he alerts the US Navy of the plot. He and Cordelia escape and they must think up a plan to stop the robbery and hope that the Navy can get to them in time to assist.

Mannering discovers that his Paris shop is being used as a front for drug trafficking.