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.

Shinichi and his Parasyte, now named "Migi," battle for control of Shinichi's arm as the number of Mincemeat Murders increases. But when an encounter with another Parasyte turns hostile, they must work together if they hope to survive.

Also known as "Symposium." Sinichi and Migi come across two more of Migi's kind - one, a new teacher at Sinichi's school named Ryoko Tamiya, has the ability to keep a low profile and adapt; while the other is an evil creature who will continue to kill humans mercilessly until the day he dies. He sees Migi as a threat to his livelihood that must be taken care of immediately.

While Shinichi and Migi are learning to work together, Shinichi's relationship with his parents (especially his Mother) is becoming more distant and strained.

A violent confrontation with a gang brings Shinichi face-to-face with a girl named Kana, who begins to take an interest in him. Shinichi's parents continue their vacation, but their trip is cut short by an unexpected attack.

Struck through the heart by his mother-turned-parasite, Shinichi survives only through Migi's efforts. After regaining consciousness, Shinichi is devastated by the realization that none of it had been a dream.

Shinichi sets out for revenge and runs into an unexpected ally. Together they team up to take down the Parasyte that brutally attacked Shinichi's family. But when Migi falls asleep, Shinichi is forced to engage the enemy on his own.

Shinichi has regained some semblance of a normal life. However, with Migi's cells dispersed throughout his body, his physical abilities have greatly increased to the point that not even Mitsuo poses a threat. Shinichi fears that the changes go beyond the physical and into the psychological. Just as peace returns to his school, a new parasite appears!

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.

The confrontation between Yuko and Hideo comes to a gruesome head when Hideo goes on a rampage throughout the school. With Satomi missing, Shinichi must hurry and find her before Hideo can get to her first.

Kana's feelings for Shinichi grows day by day, and so does Shinichi's apprehension about Kana's ability. As ordinary life returns, Satomi struggles with her mounting suspicion that something has changed in Shinichi. Meanwhile, a new parasite appears to wreak havoc in a local gang hideout. Everyone's lives are about to get shattered yet again.

Migi says that Kana can emit a rather weak signal to other parasites so they can also seek her out. Shinichi tells her the truth while Migi is sleeping but things go unexpectedly.

Shinichi returns to the crime scene to try to keep the feeling of mourning for Kana. He is followed by a human private investigator hired by Ryoko Tamiya and Migi is caught on camera.

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.

Although he still has his doubts, Kuramori reluctantly decides to help Shinichi hunt down Reiko and her group of Parasytes. But when a strange man appears, Shinichi soon goes from the hunter to the hunted.

A powerful Parasyte unlike any Shinichi has ever seen before attacks, and soon Shinichi and Migi are running for their lives. Meanwhile, Kuramori's search for the truth puts his family in harm's way.

Having determined Tamura to be a threat, Kusano's group moves to eliminate her. Meanwhile, Kuramori moves to avenge his family's death by abducting Tamura's baby. With her baby in his arms, Kuramori faces Tamura and threatens to throw the infant down a steep slope--what will Tamura do?

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.

Investigators continue to hold Shinichi, certain that there is something he is hiding. In an effort to uncover his secrets, they soon bring him face-to-face with the serial killer, Uragami, who has a unique ability.

The police commence their operation to take out Hirokawa and the remaining Parasytes. However, plans go awry when the Parasytes attempt to fight back. Things take an even more gruesome turn when Gotou appears on the scene.

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?

Motivated by Satomi to stay alive, Shinichi resolves to fight Gotou. With Migi's support, they press their attack in an isolated forest. However, they fail to finish Gotou at a critical juncture, causing Shinichi to lose Migi and flee in defeat.

With no chance to survive, Shinichi pulls off a desperate move.

An unexpected but familiar enemy surfaces and Migi gives a final farewell.