Music professor Alec Guiness rents a London flat from sweet old lady Katie Johnson. He tells her that, from time to time, several other musicians will visit in order to rehearse. In truth, Guiness can't play a note, nor can his vistors: he's a criminal mastermind, holding court over a gang of thieves, including the likes of punkish Peter Sellers, homicidal Herbert Lom and punch drunk Danny Green.