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, the parasyte in his right arm decides to take on the name of Migi and unbeknownst to him, he is about to meet another parasyte for the first time.

When Migi warns Shinichi about the existence of another parasyte, he is finds that it's in his own school, so he goes to a schoolwide meeting, where the staff present a pretty new teacher.

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

Running late to school, Shinichi notices Nagai having difficulty with some punks and when he tries to help, he 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.