En route to conquer a Great Labyrinth with his allies and former classmates, Hajime Nagumo is sidetracked by a tragedy that befalls his party member Shea Haulia. After encountering a group of rabbitmen caught in an ambush, Nagumo learns that members of Shea's Haulia Tribe have been abducted by soldiers of the Hoelscher Empire. To alleviate the anxiety of Shea, whose father may be in mortal danger, Nagumo decides to head toward the empire and rescue the missing Haulias. After arriving at his destination, Hajime is caught up in political turmoil between the empire and the Heiligh Kingdom. As princess Liliana S. B. Heiligh tries to formalize an alliance with the empire to protect her weakened kingdom against demon attacks, the Haulia Tribe plans to assassinate Emperor Gahard D. Hoelscher to free all rabbitmen from the empire's threat. Determined to protect his allies at any cost, Hajime will need to ignore all political considerations and unleash his unstoppable power on anyone who may stand in his way.

The fierce confrontation among the Kanto high schools reaches new heights with the awakening of dragons in the Kyousho and Seito Academy's leaders, Moutoku Sousou and Gentoku Ryuubi. However, the power balance between these two institutions and their main opponent, Nanyo Academy, is jeopardized when Shimei Ryomou seizes a mysterious dragon orb in China. It is said that the orb grants its strength to a dragon wielder, and Shimei hopes to save Nanyo Academy's leader Hakufu Sonsaku from the adverse effects of her unstable gift. Meanwhile, Moutoku's powers completely consume him, and he loses control over his own body to the Demon King's soul, whose unlimited ambitions endanger the stability of the region and the safety of his allies. In the face of growing tensions to claim the dragon orb, Hakufu and Gentoku will have to put aside their differences and work together to thwart Moutoku's demonic plans.

The ordinary life of 17-year-old otaku Hajime Nagumo is disrupted when he and his classmates are summoned to a fantasy world and tasked with saving mankind. While his classmates are gifted with impressive abilities useful in combat, Hajime is belittled for only gaining an inferior transmutation skill that lacks any real offensive power. During an expedition in the Great Orcus Labyrinth, Hajime is betrayed by one of his classmates, plummeting him to the bottom of an abyss. Though he survives the fall, Hajime is faced with menacing monsters and misfortunes that send him spiraling into a grim nightmare. Desperate to live and return home one day, he resolves to fight for his survival—only to meet an imprisoned vampire he names Yue, who is also seeking to escape the labyrinth. Taking an interest in him, Yue and a few others along the way accompany Hajime on his journey to find a way back home, while steadily transforming from commonplace to the world's strongest.

Haruka Shinozaki has been interested in the class representative, Akiho Kousaka, since his first year in high school. She is attractive, good at sports, and is an all-around model student. Since they are in the same class this year, Shinozaki decides to confess his feelings—and, to his shock, Kousaka agrees to be his girlfriend! However, he finds that Kousaka is a bit stranger than he first thought: this seemingly perfect girl has never been in a relationship. But even though she is inexperienced, she vows to please Shinozaki in every way she can... such as learning multiple sex positions or his fetishes. Shinozaki tries to assure her that her studies into such subjects aren't necessary, but Kousaka devotes herself to making him happy in more ways than one.