Cycle of Destruction

After Mark starts behaving oddly, the Doctor and Anya find themselves on a strange planet with their friend missing. Searching for him they find their way to an isolated research base. The corridors show signs of animal incursions and the scientists are behaving strangely. But a big surprise is waiting for them.

Because this is where Mark Seven came from. They're about to find out who he is. But is that information too dangerous to know?