After a hard night's drinking, old friends Lt. Commander Peter Fraser RN (Brian Reece) and Parliamentary Private Secretary to the First Sea Lord, Tom Baker M.P. (David Tomlinson) somehow and up swapping hotel rooms - and clothes. The next morning, still in a drunken haze, Tom the M.P. is accidentally mistaken for his navel friend - and is put in charge of the fighting ship HMS Sherwood! There's more confusion when Peter the ship's captain is mistaken in turn for the Parliamentary Private Secretary and whisked off to Admiralty house for high level political debates on the future of the Navy! Meanwhile, a little accident involving a live torpedo and the First Lord's barge gets poor Tom sent to a naval psychiatric hospital - where his insistence that he's not a navy captain but an M.P. just makes him look crazier! Doddery old Admiral Godfrey (A.E. Matthews) hasn't got a clue what's going on, so it's left to his sharp-witted granddaughter Jane (Eunice Gayson) and her friend Susan (Peggy Cummins) to save the day - and the reputation of the Navy!