The following version of this book was used to create the guide: Ballard, J.G. The Drowned World. Liveright, 2012.

The opening of The Drowned World finds its protagonist, Dr. Robert Kerans, living in the upper floors of an abandoned hotel called The Ritz. It is the year 2145, and the world has experienced a cataclysmic climate apocalypse as a result of fluctuations in the thermal energy released by the sun. As a result, most of the world—aside from some remaining land near the poles—has been submerged beneath the water. Kerans is a member of a military research dispatch from a settlement in Greenland that is commanded by a man named Colonel Riggs. The dispatch has been studying three linked lagoons above the city formerly known as London for the past several months. Riggs informs Kerans that the unit is likely to abandon the area soon as a result of rising temperatures and a feeling that the city is likely not salvageable.

Kerans is unsure of how to feel about Riggs' desire to leave the lagoons, in large part because he knows his friend (and romantic interest) Beatrice Dahl is unlikely to want to abandon her research. She has been living in a luxurious penthouse suite across the lagoons from The Ritz, and indeed, when Kerans visits Beatrice and informs her of the unit's intention to leave, Beatrice suggests that she will stay behind. Kerans' friend and colleague, Dr. Alan Bodkin, expresses a similar desire to remain at the base and continue his studies. Bodkin has been ministering to one of Riggs' soldiers, Hardman, for the past several weeks; Hardman has begun to have vivid, overwhelming dreams about the sun.

The following day, Hardman disappears. Riggs orchestrates an exhaustive search for him, and they manage to locate him in some silt flats outside the lagoons, making his way southward toward the heat and the sun. When they try to apprehend Hardman, he manages to escape. His actions are barely recognizable as human, and he cannot be reasoned with.

When the time comes for Riggs and his men to leave and return to the northern base, Kerans indeed elects to stay behind with Beatrice and Bodkin. The three of them live in relative isolation from one another, continuing their research and growing more and more accustomed to the tropical climate. All of them begin to have the same vivid dreams as Hardman. Their lives are peaceful until, one day, a large hydroplane arrives at the lagoon, piloted by a man in a white suit. After some debate, the three researchers decide to make their presence known to the man and the large group of sailors who arrive with him. The man introduces himself as Strangman, and begins inviting Beatrice, Bodkin, and Kerans to events aboard his large boat.

Kerans is immediately distrustful of Strangman, who appears to harbor an attraction to Beatrice and who frequently swings between welcoming accommodation and intimidating autocracy. Kerans also struggles to understand why Strangman's men are loyal to him. Bodkin believes that Strangman is a pirate of sorts, and his actions indeed support this theory; he and his men immediately begin making patrols of the lagoons, diving below the surface in order to explore the sunken buildings and extract artifacts from them. Strangman invites Beatrice, Bodkin, and Kerans to a diving mission at an old planetarium, and Kerans ends up volunteering to perform the dive. While beneath the surface, his line gets caught through his own decision-making, and he nearly drowns.

Strangman embarks on his next endeavor: draining the lagoon. After he does so successfully, he and his men begin plundering the city even further. This perturbs the three researchers, who are afraid that their way of life at the lagoon is going to be destroyed. Eventually, Bodkin sneaks away to blow up the small dam that Strangman's men have built to keep the water at bay. Bodkin fails to blow the dam, and he is summarily executed. Beatrice is captured and kept aboard Strangman's boat, while Kerans is lashed to a throne and circled by Strangman and his men in a kind of ritual.

After several days of torture, Kerans finally manages to escape while Strangman and his men are drunkenly looting the city. After escaping, he forms a plan to rescue Beatrice. Though he manages to free her, Kerans and Beatrice are surrounded by Strangman's men as soon as they exit the ship. Just then, Riggs arrives with a unit of soldiers and ambushes Strangman. Kerans and Beatrice are freed, and the pirates are subdued.

In the aftermath of his rescue, Kerans comes to learn that Strangman will likely be commended for the scientific achievement of draining the city, and that he and Riggs have reached a fragile peace. Strangman invites Kerans and Beatrice to a farewell party aboard his ship. Kerans manages to slip away, just as Bodkin did, and this time successfully blows the dam keeping the lagoon dry. It refloods, and Riggs begins hunting for Kerans. After bidding Beatrice a quick goodbye, Kerans escapes to the silt flats and begins moving south.

On his trek southward, he encounters a dying man propped against an altar, blinded by the sun. This is Hardman; he is barely clinging to life. Kerans takes care of him for a few days before waking up to find him gone. Then Kerans continues marching south.