Enter, with the Doctor, Anji and Fitz, an Empire where the laws of physics are quite preposterous -- nothing can travel faster than the speed of light and time travel is impossible.

A thousand worlds, each believing they are the Centre, each under a malign control of which they themselves are completely unaware.

As the only beings able to travel between the worlds instantaneously, the Doctor and his friends must piece together the Imperial puzzle and decide what should be done. The soldiers of the Ambassadorial Corps are always, somehow, hard on their heels. Their own minds are busily fragmenting under metatemporal stresses. And their only allies are a man who might not be quite what he seems (and says so at great length) and a creature we shall merely call... the Collector.