[1] Jericho is stationed in Bletchley Park, the British cryptology central office, and is worked to the point of physical and mental exhaustion.

In February 1943, Tom Jericho, a gifted cryptanalyst at Bletchley Park, is recuperating in Cambridge from a nervous breakdown brought on by the pressures of work and the breakup of his relationship with Claire Romilly, a cipher clerk.

Back at Bletchley, Jericho is still infatuated with Claire and makes his way to her lodgings, only to be told by her flatmate Hester Wallace that she is not home.

Jericho discusses the Enigma lockout with Jozef "Puck" Pukowski, an Anglo-Polish cryptanalyst who fled Poland after the invasion by Germany and so left his family behind.

Hester and Jericho bluff their way into a signals-receiving station and purloin copies of the full set of undeciphered signals.

Back at Bletchley, Jericho joins the effort at deciphering contact reports and eventually produces a 'menu' for the cryptanalytic 'bombes' to work upon.

Recuperating in hospital, Jericho is told by MI5 officer Wigram that Puck, outraged by British authorities' decision to hush up the Katyn massacre perpetrated by their Soviet allies in which his father was murdered, had been preparing to defect to Germany to bring proof that Bletchley had broken Enigma.