Naral Island exists in a world of adventurers and magic. Each generation, a Demon Lord rises to plague the land, and a Hero is born to subdue him. For countless centuries, this cycle has repeated with no end in sight. The latest Hero, Juulia "Yusha" Charldetto, has nearly completed her valiant campaign alongside her party members: the responsible priest Seiran "Seira" Élénoir, the enigmatic mage Meiza "Mei" Endust, and the hyper-energetic warrior Fai Fai. During the final battle against the Demon Lord, Yusha's party attempts a risky spell to cast their enemy into the drifts of time. However, the incantation goes awry, sending Yusha and her friends back to a time before the Demon Lord, before Yusha becomes the Hero, and before the party had even graduated as adventurers. With their memories of the future erased, the four girls restart their ambitions to become the Hero's Party, aspiring to defeat the Demon Lord. However, in a sudden twist of fate, the Demon Lord is also sent back in time with her memories intact. Reduced to the form of a little girl, the Demon Lord takes the name Mao and infiltrates the adventurers' school as a teacher, planning to stop Yusha before she becomes a hero. Thus begins the story of Yusha and her friends in their quest to defeat the Demon Lord, unaware that the one they seek is right by their side.