For two weeks now it has been the same message again and again, and it's getting stronger; death and destruction, the end of all things, Armageddon.
In an attempt to lift the Doctor out of his irritable and erratic mood, Bernice suggests he investigates the mystery of the Seven Planets — an entire planetary system that disappeared without trace several decades before Bernice was born.
One of the Seven Planets is a nameless giant, quarantined against all intruders. But when the TARDIS materialises, it becomes clear that the planet has other visitors: a hit-squad of killer androids; a trespassing scientist and his wife; and two shape-changing criminals with their team of slaves.
As riot and anarchy spread on the system's colonised worlds, the Doctor is flung into another universe while Bernice closes in on the horror that is about to be unleashed — a horror that comes from a terrible secret in the Time Lords' past.