A boy named Sim is born on a planet where humans were shipwrecked long ago and with extreme hot/cold at day/night. The environment speeds their heart rates, causing them to age rapidly--each only lives for 8 days. His father, cynical about the quick inevitability of death, tries to kill him as a baby, but his mother stops him. He has an older sister named Dark. He learns, even as a baby, seemingly by mental osmosis, picking up fragments of knowledge from his parents' minds. With the freezing nights and the burning sun, there's only a small window of time each day during which they can be outside.

Sim makes an enemy of another boy, Chion, who picks a fight with him. He develops a crush on a girl, Lyte, who Chion also desires. Sim becomes obsessed with the scientists, who live deep in the caves and are trying to learn how to reach the one still-functional ship from the wreck, too far away to ever make it to before the sun rises and burns anyone outside. The other people, including his sister, scoff at the scientists' attempts and believe people should just enjoy their short lives and live for love. Sim finds the scientists' and requests to join them, though their short lives makes their research progress at a crawl.

Two days later, Chion finds him and convinces him to join them in an ongoing war with people who live in caverns in another cliff. A mineral in the other caverns allows for a longer life--twelve days instead of eight. Sim also is tempted by the idea that the other caverns are closer to the ship. Lyte insists on coming to war with him, since she says Chion plans for him to die so she'll be his mate.

The war seems impossible, given their disadvantaged position attacking the cliffs. As the sun is rising and the others retreat, Chion bludgeons Sim's ankle with a rock so he won't be able to make it home in time. Still, Lyte insists on staying with him and Chion gives up on her.

Sim yells up at the cliffs challenging one man to come down and battle for his cavern. He wins, passes out, and wakes up in the cavern with Lyte, the two of them saved from the sun by the people of the caverns.

That night, the two of them make a run from the caverns to the ship. It doesn't look like they'll make it in time, but the rain creates a river which they let carry them to the ship. They make it just in time and pass out for days inside the ship, where their bodies slow to a more normal lifespan. When Sim finally moves, he is shocked to see they are still young.

Sim uses a blaster on the ship to blast a gulley to their home cliffs, so they're guaranteed a river with which to bring others. The only way for Sim to make it there in time is to start during the freezing hours of the night, so he leaves a line of flame from the ship running that he can run parallel to.

When he makes it home, he meets Dark and Chion as elders. 1000 follow him back to the ship, only half of them making the journey alive. When Sim's back on the ship, Lyte says to him: "I dreamed I lived in caves in a cliff on a cold-hot planet where people grew old and died in eight days and were burnt." They take off into space.