Humanity has spread to every star within 500 light-years of its half-forgotten origin, coloring the sky with a haze of habitats. Societies rise and fall. Incautious experiments burn fast and fade. On the fringes, less modified humans get on with the job of settling a universe that has, so far, been empty of intelligent life.

The ancient starship But the Sky, My Lady! The Sky! is entering orbit around a promising new system after a four hundred year journey. For its long-lived inhabitants, the centuries have been busy. Now a younger generation is eager to settle the system. The ship is a seed-pod ready to burst.

Then they detect curious electromagnetic emissions from the system's Earth-like world. As the nature of the signals becomes clear, the choices facing the humans become stark.

On Ground, second world from the sun, a young astronomer searches for his system's outermost planet. A moving point of light thrills, then disappoints him. It's only a comet. His physicist colleague Orro takes time off from trying to invent a flying-machine to calculate the comet's trajectory. Something is very odd about that comet's path.

They are not the only ones for whom the world has changed.

“We are not living in the universe we thought we lived in yesterday. We have to start learning the world all over again.”