Kiska, a boy persecuted by his village because his hair color resembles that of a family that once betrayed humanity, has only his childhood friend Namia to rely on. One day, Namia is murdered by three men, and the villagers frame Kiska for the crime, sentencing him to an inescapable S-class dungeon. With no combat skills or magical ability, he expects to die quickly. As a high-level monster attacks him, a mysterious voice grants him the skill Save & Reset, which lets him rewind time to the moment he last saved whenever he is killed. Armed with this power, Kiska begins a grueling cycle of death and rebirth inside the dungeon.

The dungeon is not only filled with lethal monsters but also with beautiful but dangerously obsessive women. The first is Ageha, the one who gave him the skill, a yandere who both helps and harms him. Later he meets Yudite, a vampire who kills him many times before he becomes her disciple, and a parasitic sword named Kaira that eventually becomes his trusted partner. Through repeated deaths and resets, Kiska learns the dungeon's secrets, forms uneasy alliances, and endures betrayals. His original goal is survival and revenge against the villagers who wronged him, but the dungeon's yandere inhabitants complicate every step.

After conquering the dungeon with Kaira's help, Kiska emerges to exact his revenge, but new trials await. A later arc sees the world reset by Ageha, sending Kiska a hundred years into the past. There he joins the legendary hero Eligion's party to fight the Demon Lord while searching for Ageha. The narrative twists through multiple timelines, deadly yandere attachments, and the relentless cycle of dying and restarting, as Kiska clings to his determination to survive, escape, and settle old scores.