Larry Niven weaves together time travel and fantasy to create an utterly unique novel on the origin of the Martian "canals.".

Hanville Svetz was born into a future to match the sorriest predictions of Greenpeace. Most of Earth's original life forms are extinct. It is Svetz's job to go back in time and retrieve them, or at least it was until his Institute for Temporal Research was transferred. Now, with a new boss obsessed with stars and planets, Svetz must figure out why the Martian canals have gone dry and what that means for Earth's future.

Because Mars was inhabited. When Svetz learns how the sapient Martian species were wiped out he realizes that Earth could soon fall victim to a similar fate. Together with his dog, Wrona, a visitor from the distant past, and Miya, an astronaut with her own complex history, Svetz must struggle to unravel a puzzle that will tax not just his rational mind but the very limits of his imagination.