In a world of adventurers and magic lies Naral Island. Every generation, a Demon Lord rises to plague the land, and every generation, a Hero is born to subdue him. For countless centuries, the cycle has repeated with no end in sight. The latest Hero, Juulia "Yusha" Charldetto, has almost completed her valiant campaign alongside her party members: responsible priest Seiran "Seira" Élénoir, enigmatic mage Meiza "Mei" Endust, and hyper-energetic warrior Fai Fai.

In the final battle against the Demon Lord, Yusha's party attempt a risky spell to cast their enemy into the drifts of time. But 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 was 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, not knowing that the one they seek is right by their side.