On a vacation in France from his nondescript job and life, John Barratt encounters a titled but impoverished French nobleman who looks exactly like him. The nobleman gets John drunk, and switches places with him to get a breather from his failing business and too-complicated life. John tries to convince everyone he is not whom they think he is, but he begins to get more and more involved with the family, including an unhappy wife, domineering mother, lonely but talented young daughter, bitter spinster sister and the expected mistress. As John gets to know them he feels he can help them with their problems, but is also becoming used to his borrowed life, which has given him a purpose for the first time.

The San Demetrio of the title is a British merchant ship in an Atlantic convoy in 1940. Disabled and left to the mercy of patrolling U-boats the crew must keep her afloat and out of harms way.

Louis Mazzini's mother belongs to the aristocratic family D'Ascoyne, but she ran away with an opera singer. Therefore, she and Louis were rejected by the D'Ascoynes. Once adult, Louis decides to avenges his mother and him, by becoming the next Duke of the family. Murdering every potential successor is clearly the safest way to achieve his goal...