Soapy Stevens visits his former partners-in-crime Dodger Lane, Jelly Knight and Lennie Price in prison. His proposal is that they steal a million pounds' worth of diamonds. And the fact that his three mates are in gaol will be the perfect alibi! Actually, the prison is more or less an old lags' paradise, under the administration of a benign governor more interested in roses than discipline. The quartet plan to escape in a Black Maria. But the arrival of a disciplinarian chief prison officer named Crout almost disrupts their plans.