At the start of the 20th Century, the Fenians have decided to rob the gold bullion stored in the Bank of England, not only for the cash, but as a political statement. To accomplish this they have imported Irish-American mining-engineer-turned-bankrobber Aldo Ray. He studies the plans of the Bank when he can get his hands on them, and makes a friend of Peter O'Toole, the officer often assigned to command the unit of Guards who make sure the Bank isn't robbed at night.
Although the there's no doubt that it's American Ray in the lead, and the wealth of British performers is a pleasure, with John Le Mesurier, Elizabeth Sellars, and the always entertaining Hugh Griffith on hand, it's most notable as O'Toole's first major role, and he owns the movie with his effete, bored Guardsman who'd like to try thinking just once; when he does, it breaks his heart.