The Doctor and Leela visit the London Zoo to find a disaster. The animals have been freed, and there are several dead animals and humans. The Doctor and Leela are attacked several times by animals. The Doctor encounters a small boy whose mother has been killed. The Doctor deduces that someone or something is making the animals act in odd ways.
They make their way to the aquarium, where the Doctor shows Leela a purple cod fish named "Stanley". He explains to Leela that Stanley is actually a former dictator from Halemida, who was exiled to Earth and forced to live beyond his normal life expectancy. It is Stanley who is controlling the animals. Stanley tries to control Leela, but the Doctor rescues her. Stanley then enters the Doctor's mind, where he finds out his planet and civilisation are gone. He lets the Doctor go. The small boy and a monkey empty the fluid out of Stanley's tank, killing him. The Doctor figures that Stanley controlled the boy and the monkey so that he could kill himself.
- The Doctor mentions Faust, Hamlet and Oedipus Rex.
- Some of the zoo animals that escaped include elephants, a lioness and baboons.
- The Doctor compares Leela to Arthur Conan Doyle.
- The Doctor and Leela are headed to Madame Tussauds.
- The boy holds a wallaby.
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