A human-built device, the Interface project, returns to the solar system after 1,500 years. The project, towed by the spaceship "Cauchy", returns a wormhole gate, appearing to offer time travel due to the time 'difference' between the exits of the wormhole (relativistic time dilation), with one end having remained in the solar system and the other travelling at near lightspeed for a century. The Qax had destroyed the solar system gate, but a lashed-up human ship (a great chunk of soil including Stonehenge, crewed by a group called the Friends of Wigner) passes through the returning gate, travelling back to the unconquered humanity of 1,500 years ago.