The story involves human mutations called "dicloniuses," who grow horns on their heads and possess telekinetic powers, in the form of invisible "arms" called vectors.<br /><br />The tendency for violence among dicloniuses varies among characters. Certain scientists contain them in a Facility in cells, where they are subject to inhumane experiments to test their abilities or where they await execution for being too dangerous.<br /><br />An infectious virus is also associated with the mutation. This virus is spread through non-lethal contact with the vectors of a diclonius.<br /><br />The main non-mutant character in the story is Kouta, who ends up becoming increasingly involved with these dicloniuses, as they seek refuge from the scientists.<br /><br />The main diclonius character is Lucy/Nyuu, a killer whose alter ego is a hapless and very cute child whose vocabulary and intelligence are quite limited.<br /><br />After a large amount of choice occurrences, the lives of these two characters become inexplicably intertwined.