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, a high school student. The hatched parasyte attacks Shinichi, but it faces unexpected resistance and fails to infiltrate his brain.

Yuko is drawn to Shimada Hideo, but gradually realizes that something is seriously amiss with Shimada's face itself. As Yuko pursues Shimada's secret, she connects the dots between him and her brother's police investigation.

Shinichi's secret becomes known to Kuramori, the private investigator hired by Tamura. Shinichi tries to confide in Satomi, but his unsuccessful attempt only drives a wedge between them. Shinichi recruits Uda's help to break the impasse.

Shinichi and Migi arrive at the park, only to find Tamura holding her child. As Shinichi struggles to quell his fury at the sight of her, Tamura unleashes a flurry of questions about the meaning of the parasites' existence and what humanity means to them.

The assault squad succeeds in eliminating Hirokawa, the parasites' leader, but they find themselves in a bind when the ultimate parasite appears. Their failure to eliminate Gotou strikes fear into Shinichi's heart--who will give him the final courage to face Gotou?

With Gotou's defeat, Shinichi's life is finally back to "normal." Though saddened by Migi's sudden departure, Shinichi moves forward with his life. However, a final threat appears...