"So what's so terrible about Perivale?" the Doctor asked as he caught up with her. Ace sighed again. "Nothing ever happens here."

Ace had wanted her homecoming to be spectacular. She had imagined the amazed greetings of her old friends, the gasps of surprise as she recounted her time-travelling adventures.

But Perivale on a summer Sunday seems the least lively place in the universe. The members of Ace's old gang have gone away — disappeared.

The Doctor has other things on his mind. What is killing the domestic pets of Perivale? Who are the horsemen whose hoofprints scar the recreation ground? Where have the missing persons been taken? Is the Doctor stepping into a well-prepared trap? And if so, can it be the work of the Doctor's old adversary the Master?

As Harvey the grocer said to his partner Len: "I'm telling you, you put a catflap in and you get just anything coming into your house."