Bell Cranell is a rookie adventurer and the sole member of his dirt-poor guild founded by the petite goddess Hestia. Adventurers come from far and wide looking to strike it big in the massive labyrinth known as Dungeon, located beneath the city of Orario. Bell is more interested in picking up girls than Dungeon-crawling, but his reality check comes sooner than expected. Saved from a brush with death by the beautiful, renowned adventurer Aiz Wallenstein, Bell falls head-over-heels and sets his sights on winning her heart, but she's not the only woman he's caught the attention of, for better of worse.

When he was a child, Murakami was friends with a girl he called Kuroneko. One day, while trying to prove the existence of aliens to Murakami, she is killed in an accident that he barely survives. Guilty of his role in the accident, Murokami decides to dedicate his life to proving that aliens exist. Then, one day, a new student with mysterious powers transfers into his class who not only looks a lot like Kuroneko, but is named Kuroha, Neko.