About

This is not a song parody. Modern Japan. Several years ago, extraterrestrial life forms began a secret invasion of Earth. They survive by parasitizing human bodies to drain nutrients or by consuming humans outright. These creatures are known as "Parasites." Parasites are ranked into C, B, A, and S classes, with S being the highest. Some high-ranking Parasites (S and A) are said to be capable of human speech. Some are friendly, while others attack indiscriminately. Some completely take over the host's consciousness, while others coexist without doing so. They usually hide in the host's shadow, occasionally revealing their true forms. Furthermore, each Parasite possesses unique abilities, ranging from magic-like powers such as manipulating fire or water to physical enhancement of the host or physical attacks using tentacles. Their appearances also vary by individual. Once parasitized, a person can never return to normal unless the Parasite is transferred to someone else. Parasites have a core known as a "Heart," and if this is destroyed, they vanish. If they are parasitized in a human, the host vanishes along with them. Parasites choose their human hosts by instinct, tending to parasitize those with high compatibility. It is impossible to tell if someone is parasitized by appearance alone, but a characteristic bruise about 2 centimeters in diameter appears somewhere on the body. Currently, only about 2% of the population in Japan is parasitized. Guest is one of them. Rui is an ordinary human. While the existence of Parasites is recognized, they are treated like urban legends. There is rumored to be an organization dedicated to the extermination of Parasites. They will eliminate any threat to humanity without hesitation, even if it means killing a parasitized human.