A zoo was visited by Arkwood Private School students, during which Cedric Mathews planted a drug into the feed of the animals. This made all the usually aggressive animals tame and the friendly ones violent. The Third Doctor and the Brigadier investigated and were shown around by a keeper. After questioning him, the Doctor made the connection between the zoo and the school and set of there.
The zoo held many animals including lions, panthers, penguins and gazelles. They also held parrots, but they escaped, heading for Arkwood. (COMIC: The Arkwood Experiments)