The Labyrinth city of Orario has a massive dungeon at its heart, filled with terrifying beasts. A new adventurer, Bell Crannel, challenges this perilous labyrinth, but not in hopes of finding treasure, but in hopes of picking up a girl.

"I don't want to lose any more of my family." Bell and Hestia are attacked by an escaped monster during Monster Philia. Bell fights to save the only family he has left, but is overwhlemed by the monster.

"Excuse me, are you looking for a supporter?" After his rapid growth, Bell reaches the 7th floor of the dungeon. He gets new equipment and is ready to go when a girl named Lilika approaches him. She is a supporter, and her job is to aide adventurers in the dungeon.

With Lili's help, Bell is progressing further into the dungeon than ever. Since he can focus on the fights, he can make more money. But Lili has a secret: she despises adventurers, and is waiting for a chance to trick Bell.

Lili leads Bell deep into a dungeon and abandons him. He's alone and surrounded by larger monsters than he's ever faced. She runs off with his knife and makes for the exit, but her guild mates are waiting for her.

At Bell's urging, Hestia reluctantly lets Lili into the party. Bell, worried about his weakness as he continues to go deeper into the dungeons, accepts an offer of help from Aiz Wallenstein, the girl he loves.

For an adventurer to level up, and truly get stronger, he has to go on an adventure. Bell travels down to the lowest of the upper floors of the dungeon, where he encounters a minotaur with a giant sword. Lili is badly injured, and Bell is unable to leave her behind. Aiz arrives again to save him, but he waves her off...

Bell defeats the minotaur and becomes the fastest adventurer to level up in history. After his success, Bell is given the nickname "Little Rookie." As he sets his sights on the middle floors of the dungeon, Ryuu tells him that if he wants to succeed, he's going to need a bigger party.

The party is exploring on the middle floors when another group leads a huge train of monsters to them. They're overwhelmed and caught in a cave-in. Cut off from the upper floors, with limited supplies and little strength left, they are forced to make a desperate attempt to reach the safe zone even deeper in the dungeon.

After a great struggle, Bell and his party finally reach the 18th floor. Aiz Wallenstein and the Loki Familia are already there, returning from their expedition to the lowest levels of the dungeon. With the danger passed, they enjoy a short break before they head back up to the town.

A massive floor boss has crashed down from the upper floors and begun tearing apart the adventurers. Aiz has left for the surface, and it's up to Bell and his friends to defeat the monster. With their escape route cut off, it's do or die if they ever want to see the light of the sun again.

A massive floor boss falls from above and traps the adventurers. The only way out is up, and to get back home Bell will have to defeat it without Aiz's help.