Mr. Potts Goes to Moscow (1952)

Potts (George Cole), a British sanitary engineer, leaves on holiday with a set of plans for a new secret weapon, which he has mistaken for his new plumbing invention. Everyone is hunting for him, including the Russians who find him and offer him a job in the Kremlin doing research (on plumbing he believes). He accepts, arrives in Russia and falls in love with Tania, a secret agent, and discovers the true nature of the plans he is carrying.