The Cybermen have overtaken the Earth, but from the depths of Torchwood emerges the greatest threat the Doctor can possibly contemplate. The Daleks! Reunited with old friends from worlds apart, the Doctor and Rose battle to save the Earth from the combined wrath of two of the Doctor's mortal enemies; but in this final conflict, who will be left standing, and what is the secret of the Genesis Ark, and how do his enemies plan to use it?