The upcoming festive special will see the return of one of the Doctor's biggest and most feared enemies - the Daleks       The Doctor is locked away in a high-security alien prison       Isolated, alone, with no hope of escape       Far away, on Earth, her best friends, Yaz, Ryan and Graham have to pick up their lives without her       But it's not easy       Old habits die hard       Especially when they discover a disturbing plan forming       A plan which involves a Dalek