Tiny creatures called "Parasites" have arrived on Earth with the sole purpose of taking over human bodies and preying on mankind. One such Parasite latches on to ordinary high school student, Shinichi Izumi. Fortunately, he is able to fight it off before it reaches his brain. But unfortunately, it is now stuck within his right hand.

Shinichi cannot reveal his terrible secret to those around him, but a strange relationship develops between himself and the creature he calls "Migi" ("Righty"). Before long, they begin to encounter other Parasites which launch savage attacks against them and endanger the life of his childhood friend, Satomi. When Shinichi's beloved mother is killed and taken over by a Parasite, it becomes an all-out war between humanity and the Parasites. But nothing is black and white in this struggle. Why were the Parasites created? Are they here to cull our numbers? The planet needs to be preserved, but we also want to protect our loved ones. Amidst this conflict of values, Shinichi and Migi have no choice but to hurl themselves into the fight against the ever more complex attack of the Parasites.