It has been 30 years since dungeons were established in Japan.
Kyoichi has been accepted into the same extremely difficult high school as his childhood fiancée, Tsubaki, but... Rumor has it that he kneels before a goblin, and he becomes the target of class teasing.
He tries to turn the situation around by exploring a dungeon, but is beaten up by a group of goblins.
Thinking of his younger sister, Koyoi, he is ready to die, but...
He is saved by a dungeon ruler named Kyoji.
This encounter changes Kyoji's destiny.

Every time you fight, you get stronger and stronger!
The Warriors' Dungeon Adventure with his sister is about to begin!