The novel begins with a prefatory note from a man named Charles Edward Prendick. Charles has inherited some papers from his uncle, Edward Prendick. On February 1, 1887, Edward was aboard a ship called the Lady Vain, which sank in the Pacific Ocean. Almost a year later, on January 5, 1888, Edward was found alone in a small boat. Edward was initially unable to provide any account of what happened to him in the interval, and it was assumed that he had amnesia due to trauma. Charles has now discovered some papers in which Edward describes what happened to him after the shipwreck, and he has decided to publish them. He acknowledges that while the described events are implausible, some factual evidence does align with what Edward Prendick claims.

The narrative switches to Edward Prendick’s account of his experiences. Edward explains that what is known about the wreck of the Lady Vain is incomplete. It is widely known that seven survivors were eventually picked up (it is implied that they had resorted to cannibalism to survive); another four men, who had boarded a different boat, were assumed to have died. Edward explains that he was one of those four; one man died attempting to board the lifeboat, so three men were left stuck at sea with very limited supplies of food and water.