The TARDIS arrives on a moon-sized asteroid orbiting two gas giants. With an amazing view, it's a chance for the Doctor, Steven and Sara to unwind after their recent adventures.
But they quickly find themselves in the midst of battle — on one side: a familiar group of space-suited soldiers — members of the Space Security Service. On the other: strange, squat aliens in body armour.
Surviving the initial hostilities, the Doctor and his friends discover that the SSS squad is on a terrifying mission. With many lives at stake, they have to venture deep inside the asteroid in search of a hideous weapon. But who can they trust in the battle against these Sontarans?