Focuses on Bernard Samson (Ian Holm), beginning with his search for the "mole" that threatens the Brahms Network in East Germany. Samson is sent to Berlin to bring out a Brahms agent. He is then sent to Mexico to try to persuade a KGB major (Gottfried John) to defect, using his childhood friend Verner Volkmann's wife Zena as bait. After it appears another traitor is working at London Central, Samson himself becomes one of the prime suspects.