Few decades have passed since humanity lost the war to a strange race of beasts that they call Monsters. The remaining survivors live in constant terror, fear and denial, as they are forced to offer their children in tribute to appease and feed the needs of their inhuman Monster conquerors. But not all of humanity has surrendered completely. A small group called the Warriors of Melos (Meros) still fight on, inspired only by a vision of a young lady known as the Melody of Oblivion, remembering an almost forgotten legend that if she is found and freed, then the Monsters can be defeated. In a bizarre world twisted by the influence of the Monsters and their human servants, the Monster Union — distorted by strong magic and extreme technology — a young high school student comes to a personal decision that he wants to be a warrior and fight back against the Monsters. With the vision of a beautiful young lady with flaming red hair, he becomes branded with the silver tattoo that is the mark of a Warrior of Melos, which grants him the power to fight against the Monsters. And so begins his journey, as he struggles to learn what it means to be a Warrior of Melos, the truth as to the Monsters and the mystery of the ethereal young lady known as the Melody of Oblivion.