The Chimpanzee, Timon, betrays the renegade gorilla, Aleron, and leads a group of soldiers to his hideout in the Forbidden Zone. But Aleron's humans overpower them. However, the gorilla army is on the way. They are making their way through a narrow canyon that connects Ape City to the Forbidden Zone.
Prisca, the female chimp, looks for Tern, the human who can speak with sign language. She comes across him with a woman and baby--his family (and I think this baby will most likely turn out to be Taylor's "Nova", as this story takes place 18 years before the first POTA movie). She finds out that Tern has been teaching all the humans there sign language--and that they are preparing for a war against the oncoming ape army.
Meanwhile, Aleron rides into the enemy camp and confronts Zaius. He says he'll return the hostages (Timon and the gorillas they captured) if the Apes will leave Aleron and the humans alone. Zaius won't go for it. So Aleron returns back to the Forbidden Zone. Then scientist Milo (Escape from the Planet of the Apes) shows up--and it turns out he's known about Aleron and his human group all along. He says the only way to prevent the battle is to collapse that connecting canyon with an explosive. And they have explosives that the humans stole. But Aleron and the humans want to fight, or so it seems, until Aleron gets the two chimps alone and says they should get everything ready as a backup plan. Aleron wants to save the humans but doesn't want to crush their hope.
This series continues to fill in the backstory of the five original POTA movies and shows why Zaius and the other apes are so afraid of the humans and the Forbidden Zone.