After two hired guns have been shot following a hostage situation CI5 discovers that they rang the Apex arms factory and that Apex employee John Copeland is being blackmailed into copying plans for a new tank and handing them over to a group of East German agents. The Professionals stalk the gang to a remote farmhouse where the girl is being held in readiness for a shoot-out.
Cowley and Doyle become perturbed when Bodie is obsessed with a motor cycle gang led by King Billy to the extent that his training is starting to suffer and Cowley refers him to a lady psychiatrist. However, there is a method in his madness as the gang is responsible for the death of one of his friends and he is after revenge.
C15 are pursuing a German terrorist cell who, having thrown a CIA agent off a balcony, are planning a new operation. Doyle captures a female member but Bodie, posing as an arms merchant, has his cover blown and is taken hostage by the gang to exchange with her. As the two groups meet at the air-field for the trade-off, the gang tie fifteen pounds of explosives to him.
Having shot a suspected drug smuggler outside her flat Doyle consoles witness Ann Holly and they become lovers, intending to marry. An informer tells Doyle and Bodie that 'the Christmas Man' is flying in a consignment of illegal drugs and, unfortunately for Doyle, it seems likely the 'Christmas Man' is Ann's father, who is put under surveillance. An enraged Doyle resigns. Will his indignant behaviour put the operation at risk and is Ann only an innocent bystander?
Andy Drake, a protege of Cowley, is arrested on suspicion of spying for the Chinese. Anxious to find out about Chinese security the K.G.B. plan to spring Drake whilst he is in transit from prison to court. Anticipating this, Cowley also intends to snatch him, but he has an ulterior motive - to catch the C15 mole who is passing on information to the Russians.
Hampered by two cocky Drugs Squad officers, Doyle teams up with Hong Kong cop Esther, who is in London trailing a Triad gang supplying drugs to U.S. army personnel in Germany, a racket supported by certain East Germans who want to destroy Anglo-American relations. Esther gets a lead in a Chinese restaurant threatened by the Triad whilst Bodie goes undercover as a down-and-out in a squat where two young junkies have died. The two halves of the operation meet for a shoot-out in a motel, with a less than satisfactory result.
Amnesiac Gerda collapses in a church before giving Cowley a garbled message about an attack on a diplomatic meeting the next day. After the ambulance taking her to hospital is attacked she leads Doyle and Bodie to a Kentish village where she regains her memory, telling them that she is a nanny and that her employers were abducted by gunmen who plan to hi-jack the meeting. CI5 have to locate the family and their captors before the latter disrupt the talks the following day.
Francisco Cabreras, teenage son of a South American president, is shot dead by members of a terrorist group opposed to his father whilst he is playing polo. Whilst CI5 get on the killers' trail after an eye-witness identifies their car, Doyle discovers that Anita, Francisco's feisty step-sister, is unknowingly dating one of the terrorists. She agrees to help in his capture before his gang claim another victim on a golf course.
The Fohn Fighter is a war-plane manufactured in Eastern Europe but it is a death trap and several pilots have been killed whilst flying it. A British reporter discovers this fact and aims to leak the story to the press but has a hitman, Van Neikerk, in pursuit. It is down to the Professionals to ensure that the killer does not get his prey and the story is published.
Drug dealer Gary shoots an addict and discards his gun. Fourteen-year old schoolboy Tony picks it up and accidentally wounds a friend. Seeking to destroy the evidence Gary abducts Tony from the school where Bodie's girlfriend Inger teaches and where Tony has hidden the gun in some stage props. Gary's heavy associates capture both lads and CI5 give chase.
Foreign diplomat Josef Merhart tips off villain Harry Walters that his embassy transports silver bullion to Tilbury docks and Walters arranges to have it hi-jacked. Mandy, the girl who introduced the two men, goes into hiding on Josef's advice but, as her flatmate Debbie is Doyle's girlfriend, CI5 are in a position to be suspicious and make the connection. Walters sends a gunman to kill Mandy so Doyle and Bodie must get to her first.
International hitman Rio arrives in London to arrange the assassination of Middle Eastern president Parsali, due to sign a vital peace treaty. Bodie and Doyle are put through rigorous training to act as Parsali's bodyguards, coincidentally crossing paths with the two English lads hired to do the killing and whose aspirations are seen to be similar to their own.
Following a botched robbery wounded gunman Albert Case and two of his gang take Doyle and Bodie and their girlfriends captive in an isolated farm-house along with one of the girls' family. Fortunately Cowley's customary impatience with the pair and their elusiveness comes to the rescue.
Working undercover as supposed hitmen out to fake the assassination of a government official, Bodie and Doyle's new partner turns out to be Jimmy Keller, who once saved Bodie's life when they were in the S.A.S. However, Bodie is concerned that Keller is in too deep with the gang of Italian terrorists whom they are shadowing. Is Keller just a maverick mercenary out to make money from both sides or a genuine danger to CI5?
Colonel Sangster and his gang are smuggling art treasures and industrial secrets out of the country and selling them on to foreign powers. Tibbs, Sangster's pilot, uses a complicated method of getting the crates with the illegal cargo past customs by taking them out of the country having misdeclared them on paper-work and, having been recalled to rectify the mistake, substituting an innocent cargo which is passed by Customs. Doyle goes undercover to infiltrate the gang.