On Naral Island, a wannabe hero named Yulia "Yusha" Chardiet and her friends, priest Ellenoar "Seira" Seiran, warrior Fai Fai, and mage Mather "Mei" Enderstto, are all studying at an adventurer school, where they receive a new teacher named Mao. Unbeknownst to everyone, Mao is actually a Demon Lord from the future who was defeated by Yusha and her friends but was inadvertently sent back in time. Determined to prevent Yusha from ever becoming a hero, Mao manipulates an ancient ruin exploration challenge and sends Yusha's group the wrong way in the hopes of getting them expelled...