David and Val must uncover the mole in British Intelligence who has been leaking the codes used by top secret couriers to the Soviets.

Former Canadian master spy David Brook, relegated to the Filing and Records Department by a boss who has little use or respect for David and his generation of agents, accidentally meets his former lover, British MI6 agent Val Koester, with whom he frequently collaborated in the "old days". When David is approached by a defecting Soviet agent with secrets to sell, he seeks Val's assistance in verifying the man's story.