The novel is set as a story that a psychic tells. She tells about Adam Jeffson and his participation with an expedition so difficult to join that his fiancée, a Countess named Clodagh, has to poison her own cousin to make room for him. The reason for this cutthroat competition is simple: A dead millionaire has left his $175,000,000 to the first person who can summit the North Pole.
Jeffson begins the story by attending church where a priest named Mackay gives an mystical sermon against the voyage where he essentially curses the trip, saying that the will of God is to punish anyone who attempts it. Jeffson isn't impressed by the nay-saying, so he boards the ship and the journey begins. He quickly notices that this journey has always been part of Jeffson's fate, and he settles into it. While exploring, he discovers a lake of spinning water with a stone in the center with inscriptions. Above it, there is a purple cloud, and he smells peaches and faints. He comes to, but as he walks back toward the ship, he notices that all the animals are dead. When he arrives to his ship, the Boreal, his entire crew is dead.
He continues alone. Along the waters, he sees that every island is covered in the dead. As he passes by ships, the crews are all dead. Back in London, he walks through streets full of dead people. He checks on Arthur Machen, but he is also dead. He writes down what happened to him.
Realizing he is the last remaining human, Jeffson loses his mind, adopting Turkish clothes as his daily wear, traveling to the world's famous cities to burn them down. Finally, he accepts one last journey for himself: He will build a huge palace for himself out of gold, on the isle of Imbros. The palace will be a temple to God and a home for the lonely man. After 17 years of work, he finishes the task and concludes that it was meaningless.
After burning Constantinople to the ground one day, he encounters a 20 year old woman, walking through the city, naked. She has no nurturing by humans, and she doesn't understand anything. He tends to abuse her as she follows behind him, sheepishly. He accepts her company, but makes her wear a veil. She quickly learns to speak, cook, dress herself, and many other civilized tasks. She speaks with a speech impediment, the consequence of her having never spoken words before. The girl discovers the Christian Bible and upon reading it, assesses that the humans who caused this apocalypse had done it for greed.
Jeffson is stuck in an impasse: One the one hand, he wishes to end the human race permanently. On the other hand, he really likes the beautiful young girl. He leaves her, going back to England, but astonishingly, she calls him on the telephone, telling him about the return of the purple cloud back to France. He travels to her, embracing her and proclaiming his love for her, deciding to try to escape the cloud of death with her. She tells him that if they have faith in God, God will not smite them. They accept their new role as the second Adam and Eve, ancestors of a new humanity.
