The Devil Inside (1961)

"To play this game", says one veteran criminal to a new recruit, "you need a cool head and an eye for angles." The same advice could apply to the art of filmmaking as well, and here's a good case in point: what could have been a routine British thriller, but with more style and intelligence than otherwise might be expected from such a familiar scenario. The new recruit is actually a spy hired by Scotland Yard to infiltrate an underground crime network, where he discovers during an elaborate jewelry heist more honor among the ring of thieves than among his fellow law enforcers. Aside from the ambiguous ending and a sometimes too relentless big city jazz score, this is first-class escapist entertainment with more than one trick up its sleeve.

...Note: the film was titled 'Offbeat' at the rare theatrical screening I attended, on the Berkeley campus way back in 1986...