The novel is presented as the transcribed testimony of Jessop, who we ultimately discover is the only survivor of the final voyage of the Mortzestus, having been rescued from drowning by the crew of the passing Sangier. It begins with Jessop's recounting how he came to be aboard the ill-fated Mortzestus and the rumors surrounding the vessel. Jessop then begins to recount the unusual events that rapidly increase in both frequency and severity. In the telling of his tale, Jessop offers only sparse interpretation of the events, spending most of the time relating the story in an almost journalistic fashion, presenting a relatively unvarnished description of the events and conversations as they occurred. He describes his confusion and uncertainty about what he believes he has seen, at times fearing for his own sanity. He eventually hears other members of the crew speak of strange events, most of which the rest of the crew pass off as either bad luck or the result of the witness being either tired or "dotty". Jessop only offers brief personal interpretation; he states that while he cannot discount the idea that the beings plaguing the ship may be ghosts, he presents his theory that they may be beings from another dimension that, while sharing the same physical space as theirs, are normally completely separated to the extent that neither dimension is aware of the existence of the other. He offers only vague, superficial suggestions as to the cause of his theorized dimensional breach.