Charles Nesbitt, an expert on chemical warfare, has a plan to flood the water supply of London with lethal hallucenogenic drugs. In order to show what he has in mind he intends to have a dummy run outside of the capital and the Professionals have to head him off.
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'.
The Empire Society, a white supremacist group run by a man called Hulton, is terrorising blacks and trying to forcibly evict them. Cowley and Doyle are shocked at Bodie's apparent colour prejudice when he meets a mixed race married couple but after Bodie is hospitalised Doyle goes undercover and joins the Empire Society, learning that a property developer called Miller is using the group to evict black tenants. However, the biggest shock comes when Miller's identity is revealed, though Bodie, grateful to black hospital staff for pulling him through, learns to overcome...