The Professionals get involved after Terkoff, a Russian K.G.B. agent, introduces young Sara Seaford "accidentally" to Sir Charles Milvain, a man tipped to be the next Prime Minister, with a view to gaining secret information from him and ultimately destroying him and his government. Sara turns out to be the long-lost daughter of a prostitute, Ann, whose pimp is not above a little murder.
Charley Turkel plans to spring his brother Henry from prison by holding the Home Secretary as a hostage - as CI5 discover when they arrest a small-time hood who has robbed Tutkel's armoury. He is to be snatched by bogus officers when he visits a police station for an award ceremony but Cowley, despite having a bullet in his leg, takes the place of the intended victim with Bodie as his bodyguard and thwarts the plan.
Hitman Georgi smuggles in a long-range gun from Athens and kills a traffic cop who stops him for speeding. Using a contact in the Greek community, Doyle and Bodie begin to track him down, along with the political group who have engaged him and have held a family hostage in a high-rise flat as they prepare to train the gun on a royal visitor at the Wimbledon tennis competition.
American senator John Jerry Patterson is assassinated in daylight in bogus roadworks in full view of other motorists. To flush out the killers Cowley prepares a newspaper story claiming that their faces have been recognised but the witnesses' identities are erroneously disclosed and two are murdered. Working with loose cannon operative Tommy, Bodie and Doyle must protect the others and identify the gang, in which they are helped by a witness who offers himself as bait.
A group of mercenaries dressed as soldiers rob a bank, escaping by plane from which they parachute out. It is a demonstration to impress smooth villain Simon Sinclair whom they coerce into employing them on another job. For Bodie their capture is more than a job for he knows them from his own mercenary days and their leader, Krivas, killed his girlfriend. Acting on information from captured gang member Benny, Cowley 'suspends' Doyle and Bodie to thwart the raid and get Krivas in their own way.
On leave with a hand injury, Bodie is in the country with his girlfriend Julia when they run across the Meyer-Helmut terrorist group. Bodie captures a gang member but he and Julia are chased by the gang and take refuge in a vicarage, where the gang kill the vicar. It is down to Cowley and Doyle, travelling blind, to locate their colleague before there are any more casualties.
The death bed confession of a former MI6 head reveals that Suzie Carter, key witness in a major corruption trial a couple of decades earlier, did not commit suicide by jumping out of a window as imagined but was pushed - by the police officers who were supposed to protect her. The case is re-opened but the officers themselves are now being targeted by an assassin. Bodie and Doyle, with new young colleague Tony, must find out who is ordering the hits and why.
Six years previously Doyle was a policeman at London's Docklands and he was involved in a shoot-out in which there were fatalities. Bill Haydon was arrested and charged and sent to prison. Now his daughter Jill has come to see Doyle, claiming that she has fresh evidence to exonerate her father and asks that the case be re-opened. Doyle gets close to her. Maybe too close to be objective. . .
CI5 agent Fraser, shadowing a drugs gang based in a bowling alley, stumbles on 'something bigger' but is killed before he can explain to Cowley. Bodie and Doyle are sent to stake out the bowl, where they find a dying man whose last word is "Swallows". He has been poisoned with plutonium by the Swallows Bowling Club, the cover for a disgruntled band of white supremacists who aim to blow London up with a bomb planted in the bowling alley. CI5 must locate it.
The CI5 team is charged with the protection of former American Secretary of state Dr. Harbinger at a peace conference and Cowley's quick thinking saves him from being shot by Middle Eastern hit-man Ramos. However the real target is Cowley himself and Ramos kidnaps a millionaire's daughter to force him to help him reach Cowley. Doyle and Bodie have to stop him.
Annie Irvine, charismatic evangelist and founder of the left-wing Workers-Christian Alliance, arrives back in England having survived an assassination attempt in Chicago and Cowley, an old flame of hers, charges Doyle and Bodie to protect her. They are highly amused to think of their boss as having had a love life. They detain a right-wing opponent of the Alliance but the real danger comes from Annie's managers, who seek to capitalize on her 'martyrdom'. Annie is saved but things will never be the same between her and her 'Georgie'.
Whilst Doyle is on a date with Kathie Mason his flat is robbed and a powerful laser gun which he was given to test is stolen. After an attempt to kill him, he and Bodie make inquiries as to who would have stolen the gun and Maurice, one of their informants, is shot dead with it. Another colleague is taken prisoner as bait to flush Doyle out in the open by the thief, Preston, a disgraced ex-cop sent to prison on Doyle's evidence and now after revenge. But who knew Doyle's routine that well that he - or she - was able to tell Preston about the gun?
After informant 'Nosey' Parker has tipped them off about the drug-dealing habits of boxer-turned-gangster John Coogan, CI5 interrogate Coogan's younger brother, Paul,who dies, seemingly after Doyle has hit him. Coogan gets his powerful lawyer to instigate a committee to determine whether or not CI5 should be disbanded but Cowley's own investigations throw light on the true cause of Paul's death as well as exposing Coogan's violent activities and confirming the need for the department's continued existence.
Israeli minister Asher Biebermann is kidnapped from outside the Festival Hall by two men known as Frank and John, whilst under police protection. He is taken to a house in a residential area where he chained to the bed. Acting on a tip-off, CI5 discover a photograph and a cassette tape in a telephone box and, using such information as they offer, locate the house, planning to break in from next door and rescue Biebermann.
When a bomb goes off in a restaurant where he is eating, causing some fatalities, Bodie assumes that he was the intended victim. In fact the real target is a seemingly inoffensive accountant who actually has been given a new identity after giving evidence against a gang of American mobsters. The chase is on as to who will reach him first - the villains, the F.B.I or Bodie, with a puzzled Doyle in tow.
Chief Constable Green operates a zero tolerance policing system bordering on fascist dictatorship. When a gay youth counselling centre is attacked by masked men one of the staff suspects the police are behind it and goes to see Cowley. Bodie and Doyle themselves become victimized after being charged with a minor traffic offence. However Green and corrupt inspector Chives are about to find out that they have picked on the wrong people - especially as Doyle has photographic evidence of police corruption.
Barry Martin was a founder-member of CI5 with Cowley and trains Bodie and Doyle in unarmed combat, but he has turned 'rogue' for money, delivering a witness who was to be kept under wraps for an upcoming drugs trial to his killers. He shoots another man dead and wounds Cowley, who ends up in hospital. Having tumbled to his plan, the lads pursue Martin to an underground car park where he eludes them, leading to a final shoot-out.
Temple-Blake building firm stands trial for corruption following dishonest warding of contracts by local councillor Webb. Its nervous accountant is murdered to stop him confessing all and jury members bullied into returning a not guilty verdict. Cowley is not happy and when Bodie and Doyle meet an under-cover fraud officer who posed as a workman to expose the company's use of sub-standard materials and got beaten up, CI5 determine to bring down all concerned.
Years earlier spy Thomas Darby sold out and defected to the Russians. Now he has returned to England. Cowley is anxious to locate him and find out from him the names of other double agents. Men with guns are also after Darby - to silence him before he can pass on any information.
Bodie and Doyle are assigned a routine bodyguard operation for a 'Mr. X', Middle Eastern diplomat Hanish, but no sooner have they picked him up than they are pursued by gunmen who seem to know every move on his itinerary, rendering two safe houses anything but safe and forcing a final showdown on a houseboat.
Bodie is delighted to re-encounter old flame Marikka Schuman, an East German film star, with whom he spends an idyllic time in his flat. However he is unaware that Marikka's husband and henchman Willis have set him up to take the rap for an assassination, causing him to go on the run from his CI5 colleagues. Although he is eventually cleared there is no happy ending for him with Marikka.