In 1965, at the age of 25, Alan Ackland is sentenced to life imprisonment for the murder of a business associate. In 1971, Sylvia Barker, lonely and depressed after a failed marriage and with two young children to bring up alone, seeks a new direction in her life and applies to become a voluntary prison visitor. Several years later their paths cross.

The lives of Bodie and Doyle, top agents for Britain's CI5 (Criminal Intelligence 5), and their controller, George Cowley. The mandate of CI5 was to fight terrorism and similar high-profile crimes. Cowley, a hard ex-MI5 operative, hand-picked each of his men. Bodie is a cynical ex-SAS paratrooper and mercenary whose nature ran to controlled violence, while his partner, Doyle, comes to CI5 from the regular police force, and is more of an open minded liberal. Their relationship is often contentious, but they are the top men in their field, and the ones to whom Cowley always assigned to the toughest cases.