Finally, the Sixth Doctor challenges Charlotte Pollard to tell him the truth. Who is she really? What is she doing in the TARDIS?

To discover the answers, the Doctor must travel back in time, beyond all known civilisations to the vast, mysterious Amethyst Viral Containment Station. But answers lie within the TARDIS too. Someone who has been there a long, long time...

Meanwhile, the Daleks have travelled back in time on their own mission, to bring them the ultimate victory they crave. But it is a mission so complex and delicate that even they know they must beware the web of time...

Who is Patient Zero? What has happened to Charley? And why have the legendary Viyrans been summoned?