There was something wrong with the TARDIS, it had started making weird noises a few hours ago and the Doctor had been forced to set down on a small, uninhabited asteroid so he could shut down her systems and investigate.
Now he was deep in her circuits, trying to track down the problem, but nothing appeared to be wrong.
“Maybe it’s gremlins,” Amy suggested.
“Can’t be; they leave obvious damage. Aha! I thought so!”
“What is it?”
The Doctor emerged with something grey wriggling in his hand. “It’s a Snock. Must have got aboard when we stopped off at Torchwood.”