Top Secret (1952)

"The story of the Diplomat, The Plumber and the Beautiful Spy"

A British Sanitary Engineer, goes 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. The Russians 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 then discovers the true nature of the plans he is carrying...