Callan is brought officially back into the Section by a new Hunter, who accompanies him and Meres to the airport collect Russian defector Bunin when he arrives on British soil. Callan doesn't believe a man like Bunin would ever defect and suspects he has actually been dispatched to investigate and even kill the new Hunter. Once under Section protection, Bunin claims he is actually here to pave the way for the defection of top government official Ivan Mereski, a potential prize catch who will only defect to the Section's top man in Russia.
Callan is ordered to investigate Joan Mather, a government laboratory scientist who has expressed concerns that her work might be used for biological warfare.
When Callan is accused of taking files from Head Office and planning to flee the country. Hunter suspects that his number one agent has 'gone over'. Callan, however, is playing a very dangerous game - one that could cost him his life.
Callan is manipulated by Hunter #2 into shadowing and befriending Polish émigré Sophia Rule in order to lure her former husband Brezhevski, previously thought dead and now a figure of some influence in the East, over to their side.
The Section is put on high alert when it is discovered that it has been targeted by an unknown party and that everyone from Hunter down is at risk of assassination.
Callan must dabble in the occult to find the truth behind a seemingly innocent spiritualist organisation 'The League of Light'. Is it a bona fide society, or do its unearthly manifestations hide more sinister activities.
When a French counter-intelligence agent Jean Coquet, a man with whom Callan was once close friends, dies in an engineered road accident, Callan is ordered to look after his widow Francine, who is kidnapped while under his protection.
When the KGB - Russian Secret Service - network in London is detected, the authorities move in for the kill. The head Russian slips through the net, and Callan is ordered to capture him.
Hunter sends Callan to retrieve a vital document which has been found in a safe of a ship during the war. But other people are interested in the discovery - people who are prepared to kill to obtain its secrets.
Callan secures work with his former commanding officer in order to investigate his involvement with a rich but politically unstable region of the Middle East.
Actor Ronald Radd returns as the first Hunter of the Section, blood-thirsty Col. Leslie. Callan is then sent to nail two minor Russian agents working from a pet shop.
Captured by the KGB, Callan is drugged, brainwashed and released - with instructions to kill Hunter. He obeys - but who actually dies? Hunter? Meres? Or Callan? Who get shot in the process?