An infection threatens to sweep through the chimpanzees, prompting the keepers to begin urgent action to save their lives. Meanwhile, the overweight orang-utans continue with their diet and the lemurs are introduced to the stump-tailed macaques before they move in together.

A pair of Mueller's gibbons develop a taste for life in the treetops, while three endangered golden-cheeked gibbons move into the enclosure next door. Meanwhile, a specialist doctor is called in when Freddy's bowel complaint fails to improve.