December 1, 2043 AD. The mysterious lifeform BABY invades Earth and takes control of a number of human hosts, parasitising them. The possessed bodies metamorphise into monsters called 'Organos,' which slaughter a great mass of humanity in a series of bloody massacres, bringing mankind to the brink of extinction. Thereafter, December 1 comes to be known as 'Extinction Day'. While battling an Organo, Elettra, the protagonist of the story, finds that her left hand has been infected by BABY. However, aside from a throbbing pain in her hand, nothing about the rest of her body changes. Elettra begins to search for other survivors, hoping to solve the mystery of BABY.