Henry Holland ( Alec Guinness) is a clerk at the Bank of England. Because of his perfect record and self-effacing manner, he is considered thoroughly trustworthy and is even assigned to accompany shipments of gold. In reality Holland has a carefully hidden desire to commit the Perfect Crime, and is waiting for the big chance. The big chance comes when he befriends another frustrated man, Pendlebury (Stanley Halloway) who has the foreign connections that Holland needs.
The result is a hilarious parody of the traditional gangster movie, which plays all the traditional tropes for laughs -- a holdup, a hostage thrown into the Thames, a French scene against the exotic backdrop of the Eiffel Tower, a car-chase. There is even a dizzying rush down the Eiffel Tower stairs that anticipates, in a comic mode, Hitchcock's VERTIGO.
Holland's paradoxical character, half 90-pound weakling and half criminal mastermind, was of course designed to exploit Guinness's talent for playing multiple personalities.
Though nobody knew it at the time, the movie would also become famous for one of Audrey Hepburn's first speaking parts, as a pretty waitress at the very start of the film.