In Yalda's universe, light has no universal speed and its creation generates energy. On Yalda's world, plants make food by emitting their own light into the dark night sky. As a child Yalda witnesses one of a series of strange meteors, the Hurtlers, that are entering the planetary system at an immense, unprecedented speed. It becomes apparent that her world is in imminent danger — and that the task of dealing with the Hurtlers will require knowledge and technology far beyond anything her civilisation has yet achieved. Only one solution seems tenable: if a spacecraft can be sent on a journey at sufficiently high speed, its trip will last many generations for those on board, but it will return after just a few years have passed at home. The travellers will have a chance to discover the science their planet urgently needs, and bring it back in time to avert disaster.