Lieutenant Mike Nicholson is operating out of Malta. Captaining the submarine Ursa, he's part of the fleet deputed to disrupt the flow of war supplies from Italian ports to Rommel's Afrika Korps. Although Ursa is small, under armed and frustratingly slow, she succeeds, on her seventeenth Mediterranean cruise, in sinking a German tank-transporter. That triumph puts Mike at the top of the league - he has now sunk more tonnage than any of his contemporaries. Promotion to Lieutenant-Commander, at the age of 28, is now on the cards. All he has to do is adhere to two rules: to stay alive, and to keep his nose clean.