One quiet evening, parasytes who devour human brains arrive on Earth. At the home of high school student Izumi Shinichi, a hatched parasyte attacks Shinichi, but faces unexpected resistance.

As Shinichi and his parasyte continue to coexist, it decides to take the name Migi. Unbeknownst to Shinichi, he is about to meet another parasyte for the first time.

Migi warns Shinichi about the existence of another parasyte, which turns out to be in his own school. When Shinichi goes to a school-wide meeting, the staff present a pretty new teacher.

Shinichi's life goes back to normal after A's killing spree, but he remains in low spirits. His mum gets worried, but he bluffs when she asks him what's wrong.

Running late to school, Shinichi tries to help Nagai with some punks and encounters a high school student named Kana.

Pierced through the heart by his mother-turned-parasite, Shinichi is near death and only survives when Migi takes drastic action.