The story begins with a dramatisation of the Italian manned torpedo raid on Alexandria (1941) in which two British battleships and were severely damaged. The British are worried that this new tactic will give the Italians naval supremacy in the Mediterranean, with the primary target for the Italians being the Royal Navy base at Gibraltar. To counter this threat, Bomb disposal expert Lionel Crabb (Harvey) is posted to Gibraltar. He creates an initially very small team of divers to intercept the Italian attacks and defuse the bombs. Meanwhile from Algeciras in neutral Spain, an Italian expert on underwater operations, Antonio Tomolino (Foà), is secretly watching the British base in Gibraltar and planning new attacks.