Naral Island is a world of magic and adventure where a cycle of never-ending battles takes place. Each generation, a new Demon Lord rises to cause chaos, and each generation a Hero is born to fight them. The current Hero, Juulia "Yusha" Charldetto, is close to achieving her mission with the help of her party members; wise priest Seiran "Seira" Élénoir, mysterious mage Meiza "Mei" Endust, and energetic warrior Fai Fai. During their final battle with the Demon Lord, Yusha and her comrades try a dangerous spell to send their foe to the past. Sadly, it misfires, sending the group back before the era of the Demon Lord, before Yusha becomes the Hero and before they graduate from adventurer school. With no memory of their future, the four girls set out once more to become the Hero's Party and destroy the Demon Lord. But, fate takes a sharp turn when the Demon Lord is sent back in time as well. Though, Mao maintains her memories and regresses into a child. She takes on the name Mao and covertly infiltrates the adventurer's school as a teacher, determined to thwart Yusha and prevent her from becoming a hero. The tale of Yusha and her companions commences as they try to defeat the Demon Lord without realizing she is in their midst.