Season 1

Hunter, head of the Section, requires the services of his best operative, David Callan. Callan had been 'retired' from the service due to his growing conscience, disrespect for orders, and concerns about the people he was ordered to eliminate. Callan now working as a bookkeeper, a job he despises, is offered to be reinstated into the Section if he kills Schneider, an international arms dealer.

Under threat of being placed in a red file, Callan is coerced into helping the Section hand over an ex-SS officer, Strauss, to the Israelis.

Callan must stop an old friend, Clarke, from going back to Africa, where he was training mercenaries, else Hunter will have Clarke killed.

Two CIA agents request the help of Hunter's Section in killing a Chinese agent, who has been trading in nuclear secrets. But his real mission in England is to kill Hunter.

Callan is lured into going to Berlin, where Hunter wants to use him as a sacrificial lamb to facilitate the planting of a British mole.

Hunter wants Callan to stop a blackmailing Lord from selling the negatives used in the blackmail to all and sundry.

When somebody beats Lonely up, and Callan finds out who it was he turns the table on Hunter.