Beneath the Planet of the Apes (1970)

After discovering that the planet ruled by apes which he had arrived on was really post-nuclear Earth in the future, US astronaut Taylor disappears while exploring the Forbidden Zone with his mate Nova. Brent, another astronaut sent after Taylor, lands on the planet and is taken to the ape city of Nova, where he's hidden by the friendly chimp scientist Zira and Cornelius. Brent and Nova are eventually captured by a gorilla named Ursus, a military leader who wants to lead a holy war on the mysterious inhabitants of the Forbidden Zone.