Dodger Lane (Peter Sellers) has it easy in Huntleigh Prison thanks to a progressive Governor (Maurice Denham) who sees time spent inside as an opportunity to improve oneself, all the better to contribute to society when they are finally released.
This offers Lane plenty of opportunities to pull the wool over his eyes and those of the warders, especially Warder Jenkins (George Woodbridge).
Dodger shares his cell with safe-cracking expert Jelly Knight (David Lodge) and the eager, less experienced Lennie Price (Bernard Cribbins). With fake vicar and their gang leader Soapy Stevens (Wilfrid Hyde-White), they come up with a perfect jewel heist.
The idea is that the convicts escape from HMP Huntleigh, effect their robbery (relieving a Maharajah of his extensive diamond collection while the stones are under military escort), then return to the prison on an iron-clad alibi.
Dodger’s girl Ethel (Liz Fraser) and Lennie’s Mum (Irene Handl) cooperate, but just as Soapy’s plans are laid, tough new warden Sidney “Sour” Crout (Lionel Jeffries) takes over from Jenkins and throws a spanner in the works.
The boys eventually get out, trick the Army convoy, grab the stones and return to jail without being missed.
The next morning they are officially released, but Crout recognises Soapy, who accompanies them to the station.
Soapy is arrested, and Dodger, Jelly and Lennie lose the diamonds on a train but hopefully turn up at a ceremony where the maharajah is weighed against the stones.
This quaint British caper comedy uses a simple plot to maximum effect.
Soapy Stevens/Rev Basil Fowler
Commander Horatio Bennett RN
Chief Police Officer Sidney “Sour” Crout
Chief Police Officer Jenkins