By Terry Pratchet
The main plot of the novel begins when an enormous tsunami carries the two central characters, Ermintrude and Mau, to the shores of Mau's home island:The Nation. Mau survives the tsunami by riding over it in his canoe, while Ermintrude is the lone survivor of the ship Sweet Judy, which is run aground on the island. Except for Mau, the tsunami has killed all the indigenous villagers of the Nation, including Mau's family. Since his people have all died before recognizing the transition of his child's soul into that of an adult's, the adolescent Mau now believes that he must no longer have a soul and rejects the gods for not sparing the lives of his family. Devastated, Mau carries all his people's corpses into the ocean, based on their religious belief that dead humans become dolphins. Ermitrude tries to interact with Mau but, in a moment of overwhelming fear, she tries to shoot him. Fortunately, the wet pistol fails to fire and Mau mistakes Ermitrude's gesture of hostility as one of generosity. Though they know nothing of the other's language or culture, the two start cooperating for mutual survival and they establish some basic communication. Ermintrude introduces herself to Mau as "Daphne" and never reveals her given name, which she has always hated.
Three other survivors of the tsunami from neighboring islands eventually arrive at the Nation, including an old cynical priest named Ataba, who calls Mau "Demon Boy" due to Mau's self-described lack of a soul and angry rejection of the gods. Later, several more survivors arrive, including two brothers who can speak English moderately well with Daphne, speeding along the language-learning process for all. Mau obsessively keeps vigil over the Nation, looking out for local cannibals who are known to attack without warning. At the same time, he frequently hears the disembodied voices of the Grandfathers. They convince Mau to relocate the "god anchors," which have been displaced by the tsunami. The god anchors are white stones that are said to "anchor" the gods from drifting away, according to tradition. Mau finds and tries to salvage the anchors. Ataba tries to destroy one of the anchors that he believes to be false and Mau, rescuing the ungrateful priest from a shark, consequently suffers hypothermia. This, mingled with sleep deprivation from his constant vigil, causes Mau to fall into a coma from which he is saved by Daphne, when she accepts a magic poison from a shamanistic old woman and travels in her mind to the land of the dead to rescue him.
Even more survivors have now arrived at the Nation and Daphne begins hearing the voices of the Grandmothers, who claim to be the neglected but more sensible, female counterparts to the Grandfathers. On the Grandmothers’ suggestion, Daphne prepares to open an ancient, closed-off crypt called the Grandfathers' cave. Mau, Daphne, Ataba, and some others enter this cave and discover that the Nation is probably the oldest civilization on Earth and that its people have made huge scientific progress with creations including telescopes, eyeglasses, and accurate star charts. Up until now, all this was apparently forgotten. After leaving the Grandfathers' cave, the group is confronted by two villainous English mutineers from the Sweet Judy. Armed with pistols, the mutineers promptly kill Ataba and take Daphne hostage. Daphne, though, craftily causes one of the mutineers to poison himself and the other to flee. In the meantime, Cox, who has since joined the cannibals and claimed himself their chief, is preparing to raid the Nation. The Nation's people coax the cannibals, when they arrive, into having the two parties' chiefs fight to the death in single combat. The cannibals are surprised when the Nation announces the youthful Mau as their chief. Cox expects to easily dispatch Mau with two pistols, but Mau remembers Daphne's earlier attempt to shoot him, and knows that gunpowder fails when wet; Mau therefore takes the battle into the lagoon, and he outwits and kills Cox. With their leader dead, the cannibals depart and release their prisoners. A few days later, Henry Fanshaw arrives in search of his daughter. As a man of scientific curiosity, he, like Daphne, is fascinated with the re-opened cave. The Gentlemen of Last Resort arrive two weeks later, telling Fanshaw that he is the next heir to the British throne and immediately crowning him king. Mau, wary of England's politics, is reluctant to join the British Empire and instead requests that the Nation become a member of the scientific society. Ultimately, Daphne feels a duty to leave with her father, and Mau remains behind on the island with his people.
Many years later, in the present day, an old scientist tells this story to two children of the modern-day Nation. He explains that Daphne returned to England to marry a prince from Holland and that Mau died of old age. When Daphne died, her body was sent to the Nation to be buried at sea so that her soul would become a dolphin. He tells them that from those days onward, thousands of scientists have visited the island, including Charles Darwin, Albert Einstein, Carl Sagan, and Richard Feynman, and that dozens of observatories have been created to learn about the stars, as the Nation has done for thousands of years. The book ends with the elder of the two children, a girl, standing guard on the beach, protecting the Nation as Mau had done years before. In the lagoon, a dolphin leaps from the sea and the scientist smiles.