Gamera vs. Guiron (1969)

When two children, a brother and sister, use their telescope to spot a UFO, they are transported to a distant planet called Terra. There, they meet Gamera, a giant turtle with the ability to breathe fire, and Guiron, a monstrous creature with a deadly weapon on its head. The children discover that the planet's inhabitants have a sinister plan that involves cannibalism and must find a way to escape and warn Earth of the impending danger.