Malta, 1942. Lieutenant Mike Nicholson commands Ursa, one of the 10th Submarine Flotilla's boats who, in their time, destroyed more than a million tons of war supplies shipped from Italian ports to Rommel's Afrika Korps in the Western Desert. German Intelligence has recently warned Berlin that unless drastic measures are undertaken, the Mediterranean and Middle East will be lost to them. But Ursa is a lucky submarine, and Mike is the flotilla's top scorer. He also has problems of a more personal nature: before leaving England about two years ago he was heavily involved in an affair with the wife of another submariner, who's now bringing his own new U-class boat, Unsung, to the flotilla ...