The mighty hero Ultraseven is crippled while defending the Earth from an invasion by the tyrannical Alien Magma. He is rescued Gen Otori, a young Ultra warrior who had fled to Earth following the destruction of his homeworld, Planet L77 in the Leo constellation. Unable to return to his giant form, the injured Ultraseven resumes his human identity of Dan Moroboshi and trains Gen to become the next Ultra Heroà Ultraman Leo!