The few remaining orphans in Saint Delfine Orphanage have discovered that they are a part of an experiment. Some are turned into monsters while others are "tested" on. Even with this knowledge, the few remaining orphans find themselves struggling to survive, not having any way to fight back. Desperate, one orphan named Samatha takes a gamble and manages to make it into the Vault, the place where the orphanage holds their most dangerous monsters that have the possibility to destroy the world. When she opens a strange door, instead of finding a horrific monster or some kind of vial for a virus, she finds a pink-haired girl whose name is Kallie.
Unfortunately, or fortunately, Kallie is a type of monster. She has a dark past and a massive grudge against the mastermind behind the orphanage, Aunt Ravean. And she will stop at nothing to get her revenge, even if it means killing every guard, every monster, every scientist, and every person in her way. Indeed, Kallie is quite the monster.