Here's a synopsis of the episode:
Orbiting a quiet backwater planet, the massed forces of the universe’s deadliest species gather, drawn to a mysterious message that echoes out to the stars. And amongst them – the Doctor. Rescuing Clara from a family Christmas dinner, the Time Lord and his best friend must learn what this enigmatic signal means for his own fate and that of the universe.
From the pictures just released, it looks as if the Doctor will be up against both Daleks and Cybermen, in both wooden and metal form. Should be an amazing episode.