Doctor Who 17x26

The Doctor, having met another Time Lord named Drax in the caves, teams up with him to try and defeat the Shadow, but Drax shrinks him instead of his Mute captor then turns the shrinking ray on himself, allowing the Shadow to claim the Key to Time. Meanwhile, everyone finds out that Princess Astra, herself, is the sixth and final segment of the Key to Time. And once it is assembled, nothing stands between the Shadow and absolute power of the Universe...except the Doctor!