Spiral Scratch

When the Doctor and Mel receive a message about the Lamprey, the Doctor is confused. He's never heard of such a thing. Mel on the other hand has, which is odd as the Lamprey is a demon from a distant planet, far, far in the future - somewhere she's never heard of, let alone visited. Meanwhile two strangers are watching every move the Doctor makes, one minute stopping calamity, the next causing it. Are they the force for good that they claim? Just as Mel thinks she's got this time-travelling business sorted out once and for all, along comes the peculiar Pierrot family to challenge everything she believes to be real...