Landing on a barren asteroid, the Doctor and his friends discover the final pages of a drama that has torn apart an empire are being played out.
Who is the man in the mask, and how are his chess games affecting life and death in his prison? What is the secret of the knights in armor that line the bleak walls of the settlement. And what is the nature of the alien ship approaching -- and what will it want when it arrives?
Soon the TARDIS crew find themselves under siege with a deadly robotic race and human traitors to defeat -- and the future of an entire stellar empire hangs in the balance: if the Doctor cannot triumph it will become a force not for good, but for evil.