Due to the frequent occurrence of cruel dismemberment murders, people all over the world fell into a state of panic, but the culprit was the "parasitic beast" that humans had never expected. After the larvae of these parasitic creatures invade the human or animal body, they aim to parasitize the brain. After the parasitism is completed, they assimilate with the brain cells and manage this "dead body". They can transform at will, can quickly absorb and master human knowledge, and their food is the same species as the parasite. One day, an ordinary high school student Izumi Shinichi (voiced by Shimazaki Nobunaga) was invaded by a strange insect and devoured his right hand. Since then, he has had to coexist with the parasite "right". Izumi Shinichi, who is honest and kind by nature, dare not tell his family and friends about this change, but his favorite person Murano Satomi (voiced by Hanazawa Kana) seems to have noticed the changes in his body. For the relatives and friends killed by parasites and the future of all mankind, Izumi Shinichi is determined to fight a desperate battle with other perfect parasitic beasts.