Agents Shinichi Kasu and Muneto fight "Demon Beasts," aliens who possess human bodies to adapt to Earth for eventual conquest. Tragedy strikes when Muneto's friend, Tsutomu, is taken over by one of these creatures and targets his love, Kayo. Despite a desperate struggle to save him, Tsutomu ultimately urges Muneto to end his life to protect Kayo, highlighting the brutal sacrifice required to defend the planet.