After a bomb goes off in the CI5 building and an operative is killed, Cowley gets a sinister phone call to say there will be more deaths, which there are, including an attempt to murder Bodie. Through an informant a man named Wakeman is incriminated - but Cowley swears that he killed him years earlier. Clearly an old associate of Wakeman is out for revenge, leading Cowley and the boys to a caravan primed with explosives.
Cat burglar Sammy Layden gets in way out of his depth when he stumbles upon a smuggling ring connected to the Arab Embassy, for which he pays with his life. Bodie and Doyle become burglars, 'backtracking' on Sammy's movements to discover exactly what he found that was so deadly, whilst Doyle has to cope with the attentions of Marge Harper.
Former CI5 operative Meredith was presumed dead but re-surfaces, apparently working for the Russians. A proposed exchange leads Cowley to go alone to a deserted aerodrome to meet the Russian Kodai and furnish him with a British passport and money. However, it is a trap and Cowley is shot, seemingly fatally.
Stefan Batak, a Bulgarian who has been spying for Britain, is released in an exchange of agents and C15 are charged with his safety as well as establishing whether his information is genuine. However, he is killed by a poison introduced into his blood stream and the Professionals must trace his killers as well as working out what part his daughter, concert pianist Anna, may have had in the death - if indeed she is his daughter.
Ex-soldier Mickey Hamilton shoots and kills doctors in the local hospital, believing them to be responsible for the death of his wife and for the brain damage of his little girl, who is being cared for by nuns at a convent. The Professionals track him down by marrying up hospital and army records and then must race to prevent him from causing carnage at an international medical conference which is being held at Wembley.
Palestinian leader Khadi is due in London for important and secret peace talks but his arrival has been leaked and a woman terrorist is shot whilst filming the talks' rehearsal. CI5 link up with ministerial secretary Frances Cottingham, who innocently leaked news of Khadi's visit to her boyfriend, who is not whom he says. Cowley gets her to give him false information but is he really the assassin?
A gun shop is raided but the guns are junked. It is part of a plot by a man named Duffy, a member of the political terrorist group the Organization to discredit the main body of the group, who have now formed a truce with CI5. He is also out for revenge on Doyle, who killed his brother, and lures him into a trap, as well as planting a bomb in a public car park.
Cowley appears to have stolen two canisters of a new nerve gas and is recorded trying to sell them to German Otto Hahn. Bodie and Doyle are asked to keep a surveillance on him though he asks them to shadow an Arab called Malik, which they suspect is purely to keep them off his trail. They trace him to a country mansion, where they are caught by Hahn's men but Cowley claims not to know them. Is he really guilty of treason or has he devised a plan to catch the real traitor?