Callan is summoned from his tedious office job to see Hunter, his former employer in 'the Department', a shadowy semi-governmental organisation that handles the jobs the official security services won't touch - bribery, coercion and, if necessary, execution. Hunter is willing to offer Callan his old position back, provided he proves he has overcome his 'weakness' - his excessive concern for his victims. The assignment is a 'red file', reserved for the most dangerous subjects, usually requiring assassination. The target is Mr Schneider, whose office is opposite Callan's own. Despite Hunter's refusal to relate Mr Schneider's crime, and his insistence that Callan must manage without the Department's help, Callan eventually accepts the assignment. Hunter instructs Meres, a younger agent who has been secretly observing the meeting, to tail Callan.
Callan bumps into Schneider outside his office, and discovers that they share a common interest: model soldiers and war-gaming. He warms to him, while noting that Schneider carries a gun. Later, he spies two policeman visiting Schneider, and learns that he is charged with exporting guns from Japan. Anxious, Callan tells Hunter that he will withdraw unless the police are pulled off. Hunter promises to deal with it.
Meanwhile, Callan illicitly obtains a gun from his former associate, the nervous and foul-smelling Lonely. Stealing into Schneider's flat, Callan discovers proof that his target has been selling guns to Indonesia, resulting in the deaths of several British troops.
Schneider invites Callan to his home, noting dryly that Callan neglected to ask the address. Callan informs Hunter that the job will be completed that evening, then, suspecting a trap, makes a recording for the police identifying Hunter and the Department. However, he is attacked by Meres, who leaves him unconscious and steals the tape.
That evening Schneider and Callan re-enact the battle of Gettysberg, watched by Schneider's beautiful Asian girlfriend, Jenny. The two policemen turn up. While Schneider is at the door, Jenny retires to her bedroom, where she is overpowered by Meres. Callan, furious, locks Meres in the room, but when Schneider returns he hears a sound and discovers Meres, holding both agents at gunpoint. Biding his time, Callan shoots him with a concealed weapon, then knocks out Meres, locks him in and leaves before telephoning the police. Furious at being set up, Callan then telephones Hunter to reject the job. Hunter instructs his secretary to change Callan's file from yellow to red.
