As captain of the submarine Ursa, Lieutenant Mike Nicholson’s mission is to disrupt the flow of war supplies to Rommel’s Afrika Korps. Although Ursa is small, slow and often out-gunned, she succeeds, on her seventeenth Mediterranean cruise, in sinking a German tank-transporter.

That triumph makes Mike top of the league – he has now sunk more tonnage than any of his contemporaries. Promotion to Lieutenant-Commander, at the age of twenty-eight, is now on the cards. All he has to do is adhere to two rules: stay alive, and keep his nose clean…