Science Ninja Team Gatchaman (1972) : 2x5

In the underdeveloped jungle country Fruginia, a village is destroyed by cavemen. Soon after, Neanderthal cavemen attack the country of Amehon, which is 200 kilometers away. They are immigrants from Fruginia who have been transformed by Galactor with the help of a special liquid. Among them is Toro, the father of young Shiro, whom Ken and the others met earlier by coincidence. Galactor plans to transform the all of humankind into cavemen, and, wants to pour chemicals into the water reservoirs to achieve this. However, the Science Ninja Team stops them. Galactor then releases a demonic energy which makes the cavemen violent and dangerous. They start to destroy the city. The mecha controlling the energy shoots down the God Phoenix. The team gets into the city, but can't confront the cavemen because they're humans. But when Toro is coincidentally reunited with his son and refuses to destroy things, the demonic energy flows backwards into the mecha. The enemy mecha starts moving by itself and ends up exploding. Consequently, the love of the united father and son has smashed the Galactor plans.