The Doctor and William arrive in London in the evening and meet a man named Damien McKittrick walking his dog, Mackie. William is shocked that the dog is allowed to relieve itself anywhere (including on the TARDIS), but Damien explains that a city ordinance allows dogs to do their business anywhere. He also explains that London is under a state of emergency and there is a curfew.
When a cat detector van passes them, they follow it to the home of Daphne Liggart. The Doctor uses fake credentials to get information: the van can detect cats' brainwaves. The Doctor realises the van must be using alien technology. When Angela Melocati and the police arrive, Daphne tells them her cat Tiddles is dead, but when they search the house and find evidence of Tiddles' presence, they try to find him, only for Daphne to throw them bodily out of the house. Melocati shoots Daphne.
When the Doctor and William meet Damien again, a mugger tries to rob Damien. The Doctor, William and the mugger see a long green worm come out of the dog, though Damien can't see it. The worm attacks the mugger, who runs off screaming. They next encounter a dog walker, and when one of her dogs poos a green worm, which the dog walker can't see, the Doctor captures it.
The Doctor and William are captured by the police, who have figured out that the Doctor's credentials were fake. One of the police officers is able to see the green worm, but it attacks him and crawls into his brain. He is now under its control and doesn't remember seeing it.
As Captain Melocati drives the Doctor and William to the station, they see Tiddles. Melocati gets out to capture him, but a group of people shoot her and capture the Doctor and William. At their headquarters, they explain they are the cat underground resistance movement. A spider-like alien tries to enter William's brain to communicate, but the Doctor fits a device to Tiddles, who can now verbally communicate with them.
Tiddles (or rather the alien controlling Tiddles) explains that two alien races have been playing a game on Earth. One race is controlling cats, the other race, the De'htrexi, is controlling dogs. The De'htrexi have started controlling humans too, and Tiddles' race wants to put an end to this practice, with the Doctor's help.
The Doctor goes to a meeting of city officials and brings a device that will remove the De'htrexi from the humans' brains. Their job done, the Doctor and William head back to the TARDIS, noting that dog owners now are required to clean up after their dogs.
- Sixth Doctor
- Damien McKittrick
- Captain Angela Melocati
- Daphne Liggart
- Dog walker
- William discovers that England has lost the World Cup.
- The Doctor was trying to take William to a Viking longship but they ended up at the Viking Laundrette instead.
- Captain Melocati uses a tazer on Daphne. The cat underground resistance movement uses an air pistol to shoot Captain Melocati.
- When the Doctor steals equipment from a fishing store, he leaves behind gold doubloons as payment.
- The mugger demands watches, phones and IPods.
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