Dungeons suddenly appeared in the world, and humanity gained levels and skills.

In such a world lives a man who works as an explorer, Yu Kanbase.

His unique skill is Immortality.

He was immortal, never aging or dying.

However, this skill had a drawback: he became unable to raise his level or acquire new skills.

Because of this, he merely couldn't die.

Other than that, he was powerless, an incompetent.

This was the assessment others had of him.

Due to a certain incident, this man would challenge an endless dungeon—commonly known as the "Eternal Dungeon."

Normally, it was an absolutely impassable dungeon.

But he, using his inability to die as a weapon, pressed on alone through the dungeon without food.

After 10,000 years.

Yu finally arrived at the deepest part of the Eternal Dungeon and obtained his desired item.

It was—

a magic item that could turn back time.

"Finally... I finally got it. I'll definitely... definitely protect you this time. Yeah, Mom."

Yu activated the item, reversing more than 10,000 years of time.

Back to the time when his family was safe.

Yes, Yu's purpose in enduring 10,000 years of solitude and conquering the dungeon was to turn back time and protect the family he couldn't save before.

This is the story of a protagonist who has returned in time, saving the family he once couldn't—and saving the world.