The Diclonius, a mutated human that is said to be selected by God and will eventually become the destruction of all of mankind, possess two horns in their heads. In addition, they have vectors, or invisible hands, that can kill their enemy in an instant. Due to this dangerous power, they have been captured and isolated in laboratories by the government.
Lucy, who is said to be the first Diclonius, manages to break free of her confines and brutally murders most of the guards in the laboratory where she was being held. She eventually gets shot in the head and drops into the ocean. She survives and manages to drift along to a beach, where two teenage cousins named Kouta and Yuka discover her. Having lost her memory, she was named after the only thing that she can now say, "Nyuu". The two cousins allow her to stay at Kouta's home. However, it appears that Lucy is not dead just yet, and the government is not going to sit back and let her run free...!