The story picks up with a young girl, Kasumi, severely injured and bleeding, having escaped a dangerous encounter in a dungeon. She's used all her potions and bandages, and her left leg and arm are badly hurt. She's managed to evade her attackers but can't return to the upper levels, forcing her to descend further into the dungeon. Kasumi reflects on the disastrous dungeon exploration, a rare collaboration with her rival friend and another acquaintance. She blames bad timing rather than anyone's fault, having sacrificed herself to buy time for the others to escape. Despite her critical condition, she reaches a treasure-filled room and collapses, her life fading.
Meanwhile, in the depths of the dungeon, a being named Yuuto, who is half-human and half-lich, lives in isolation. He finds a rare fire lizard and uses its flame to light his way, his body regenerating from the burn. Yuuto laments his cursed state, unable to fully become an undead creature and longing for sunlight, yet fearing it. He is accompanied by a loyal skeleton companion, whom he treats as a tool. Yuuto notices an unusual airflow, a phenomenon he last experienced when he was first trapped in the dungeon. Believing a passage to the surface has reopened, he rushes towards it, easily dispatching monsters along the way. He arrives at a treasure vault, the same one Kasumi had stumbled into, and finds her near death.
Yuuto, possessing the ability to sense life force, realizes Kasumi is still alive. He speaks to her, and she weakly identifies herself as Kasumi. Yuuto, having been cursed years ago by a lich and trapped in the dungeon, explains his situation and offers to save her, but warns it will involve a transformation. Kasumi, desperate to live, agrees. Yuuto uses his life force to heal her, a process that causes her immense pain but ultimately revives her, though with some changes. He then prepares to defend the treasure vault from approaching monsters, easily defeating them with his enhanced abilities.
Kasumi awakens to find herself in the treasure vault with Yuuto and his skeleton. She's initially terrified but grateful for being saved. Yuuto reveals he's been trapped for fifty years, having been cursed by a lich and becoming a half-lich himself. He explains the passage to the surface has reopened and he wishes to return to the human world to contribute. Kasumi, a dungeon explorer and streamer, agrees to help him, and they form a partnership. They decide to broadcast their journey to the surface, a decision that causes a stir among viewers and authorities.
Yuuto, accompanied by Kasumi and his skeleton, begins his ascent. They encounter various monsters, which Yuuto dispatches with ease, showcasing his immense power. Kasumi, a skilled explorer herself, uses her knowledge and equipment to navigate the dungeon. They learn that the world has changed significantly in fifty years, with magic now integrated into daily life and dungeons becoming a regulated industry. Yuuto, a relic of a past era, struggles to adapt to the new world but is determined to contribute.
They eventually reach the surface, greeted by high-ranking explorers and officials from the Labyrinth Ministry. Yuuto's identity as a legendary figure from fifty years prior is confirmed, and he is offered a special permit and support to reintegrate into society. He agrees to cooperate, aiming to use his abilities to protect humanity. Kasumi, now his partner, is tasked with helping him adjust to the modern world. The narrative sets the stage for Yuuto's re-entry into a world he fought to save, now vastly different from the one he left behind.