The 39 Steps.

Richard Hannay, a mining engineer on holiday from the African colonies, finds London socialite life terribly dull. Yet it's more than he bargained for when a secret agent bursts into his room and entrusts him with a coded notebook, concerning the impending start of World War I. In no time both German agents and the British law are chasing him, ruthlessly coveting the Roman numerals code, which Hannay believes he must personally crack.