Shinichi Izumi's life takes a turn when a creature called a Parasyte attempts to infiltrate his body in order to take control of it, but it ends up burrowing into his right hand and both are forced to cooperate.

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.