Alec Guinness plays Professor Marcus a self-styled criminal 'mastermind' who heads up a team of incompetent villains (Cecil Parker, Herbert Lom, Peter Sellers and Danny Green) who rent rooms pretending to be classical musicians in order to rob a bank by tunneling underground and are accidentally thwarted by their little ol' landlady, Mrs Wilberforce (Katie Johnson).
This black comedy is widely regarded as one of the classic contributions of Ealing Studios to British 1950s films. That is true, though it is neither particularly black nor comedic, at least by today’s standard. Its real appeal is now largely nostalgic with its projection of the constrained English cup-of-tea quaintness and self-regarding sense of propriety of post-war, post-imperial Britain, with Guinness exemplifying the anonymous “little man”, as he did in so many Ealing films of this period. If you're well-disposed to this sort of indulgence you'll enjoy this, if not once the heist is over there'll be little to hold your attention.
FYI: The Coens brought the film up-to-date with their 2004 version of the same name.