The whole world is about to change one quiet evening with the arrival of parasytes who devour human brains; one of their eggs lands at the home of Izumi Shinichi, and attacks him.

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.

With the number of murders increasing, a new parasite appears in the guise of substitute teacher Ryoko Tamiya.

A confrontation with a gang brings Shinichi face to face with a girl named Kana, who takes an interest in him; Shinichi's parents' vacation is cut short by an unexpected attack.

Shinichi is near death and Migi takes drastic action to ensure his survival; Shinichi learns that his father is still alive.