I've never really connected with Peter Sellers’s movies, yet this one manages to be pretty good. While behind bars, he’s enjoying a comfortable existence alongside his two cellmates during their sentence when an old acquaintance—who has a more significant role for the likable Wilfred Hyde-White—shows up disguised as a clergyman to offer them spiritual guidance. It's then that he tells "Dodger" (Sellers) about his scheme: to have them break out of prison just long enough to steal a large sum of diamonds and return, creating a flawless alibi! He and his companions "Jelly" (David Lodge) and "Lennie" (Bernard Cribbins) dismiss it as unrealistic, particularly when the rather clumsy, gardening-minded prison governor (Maurice Denham) discovers that he’s been replaced by a strict new warden named "Crout" (Lionel Jeffries)—a man set on imposing order in this laxly managed institution. What follows is a sequence of amusing adventures that I imagine could have influenced the later BBC series "Porridge." Jeffries and Denham offer solid performances as the story, however silly it may be, allows for some memorable characters to emerge and provides dialogue and situations that elicit occasional chuckles. I must admit, I was somewhat let down by how things concluded—but as far as crime comedies go, this is genuinely enjoyable and still deserving of a viewing.

Source on TMDB