The story involves human mutations called "dicloniuses", who grow horns on their heads andpossess telekinetic powers, in the form of invisible "arms" called vectors.

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 were they await execution for being too dangerous.

An infectious virus is also associated with the mutation. This virus is spred through non-lethal contact with the vectors of a diclonius.

The main non-mutant character in the story is Kouta, who ends up becoming icreasingly involved with these dicloniuses, as they seek refuge from the scientists.

The main diclonius character is Lucy/Nyuu a killer whose alter ego is a hapless, and very cute child whose vocabullary and intelligence is quite limited.

After a large amount of choice occurences the lives of these two characters becomes inexplicably intwined...