The Graves of Mordane

Convinced that the Doctor is Varlos, the Agent demands the return of the Eternity Crystal. But what happened on Mordane to make the Crystal too dangerous to be left on the Moon? The Doctor has to convince the Agent that he is not Varlos and doesn't know what happened on Mordane, then escape with the TARDIS and find out. The Doctor decides that the Eternity Crystal must be destroyed but before he can the Agent and the Crystal vanish. He has to find the Agent, which leaves the Doctor only one option, and he doesn't like it...