This WWII action drama is set in the Mediterranean during 1941 after two British battleships, the Valiant and Queen Elizabeth, were sunk in Alexandria harbour by explosives placed under their hull. The torpedoes were carried on underwater 'chariots' operated by frogmen of the Italian 10th. flotilla. Gibraltar, the gateway to the Mediterranean, was their next target. Lieutenant Crabbe (Laurence Harvey) is assigned to deal with the new threat and protect the ships of the convoy at anchor in the bay.