The author sets out from the Pillars of Hercules into the Western Ocean with fifty companions, only to be caught in a violent tempest that carries their ship through the air for over two months. They arrive at a shining island in the sky and find themselves embroiled in a cosmic war between Endymion, the king of the moon, and Phaethon, the king of the sun, who are fighting over the colonization of the morning star. Armies consisting of bizarre hybrid creatures, giant spiders spinning a web between celestial bodies, and men riding monstrous vultures clash in an epic aerial battle before a peace treaty is finally established.
Leaving the celestial realms, the travellers eventually descend back to the sea, where they are promptly swallowed whole by a colossal whale fifteen hundred furlongs in length. Inside the monster's vast interior, they discover an entire hidden region inhabited by various tribes of eccentric sea-dwellers, including eel-like men and lobster-faced creatures. They befriend an old Greek man named Scintharus and his son who have been trapped inside the beast for decades, and together they wage a successful war against the hostile native factions to secure their survival.
After spending nearly two years within the whale, the group finally devises a plan to kill the beast and prop open its jaws with timber beams to make their escape. Once back in the open ocean, they navigate through wondrous and bizarre domains, including a sea of milk that forms a massive cheese island, and a floating forest of unrooted trees. They encounter the Phellopodes who walk upon cork feet and the deceptive Onosceleans, women with donkey hooves who lure travellers to their deaths.
The narrative eventually leads them to the Islands of the Blessed, where they meet historical figures, philosophers, and mythical heroes from antiquity, engaging in philosophical debates and observing the daily life of Elysium. They witness athletic games, attend grand banquets, and converse with Homer about the true origins of his epics and the validity of his disputed verses. However, after an attempted elopement involving Helen of Troy causes a stir, the travellers are politely asked to depart.
Resuming their voyage, they pass the dismal island of the damned, where famous liars and falsifiers of history endure eternal torments, and visit the shadowy, dream-filled city of sleep. They survive further attacks from gourd-riding pirates and giant halcyons before finally reaching the shores of the opposite continent. The work concludes on the brink of their exploration of this new terrestrial world, having delivered a relentless cascade of fantastic inventions.
