In a world where monstrous calamities fighting against humanity due to advanced technology, and a world where only girls became Valkyries as the protector of humanity, there's one boy who had a dream of fighting for humanity, but due to his gender status and have no Honkai resistance, he can't. So he stumble upon thaumaturgical knowledges, and through it he sees as an alternative mean to fight against the Honkais. But one terrible day happen, causing him to have an amnesia, making him not remember anything related to the powers he possessed. But, the amnesia doesn't last forever, as one incident happen. Find out as the boy, Y/n Horizon, become the hero once again, but to do that he must become a student of St Freya High to get a Valkyrie license.