Hinata Sunohara, a high school student living alone after losing his parents, once led a very different life. He was summoned from Earth to a fantasy world, where he was tasked with defeating a Demon Lord. As part of a hero's party, he fought alongside a priestess, a cat-eared rogue, and a powerful dark elf named Mariabell. After successfully completing his mission, Hinata was abruptly sent back to his original world without a chance to say goodbye.

A month passes, and Hinata tries to settle back into the mundane routine of being a student in Japan. That peace is shattered when Mariabell suddenly materializes inside his apartment, completely naked. Despite a deep-seated hatred for humans stemming from past trauma when she was almost assaulted, Hinata had saved her and shown her kindness, which transformed her worldview. Now, she has crossed dimensions to fulfill a single promise: to be with him forever.

Desperate and obsessive, Mariabell immediately moves into his home. While generally caring, her love is violently possessive. She becomes murderously jealous of any girl who looks at Hinata, particularly his kind-hearted classmate, Sakura Mochida. Mariabell constantly tries to seduce Hinata, viewing him solely as her partner and reacting with lethal intent toward rivals. Sakura, who initially harbored a crush on Hinata, quickly realizes she cannot compete with a yandere dark elf and instead becomes Mariabell’s first human friend, bonding with her over shared interests in manga and fantasy tropes.

The story follows the reverse isekai chaos of Hinata teaching a medieval dark elf about modern life while surviving her constant advances. He must also navigate the fact that she is not the only visitor from the other world. The priestess Cecile and the catgirl Mei eventually cross over to keep an eye on the rift between worlds and to supervise Mariabell, though they too become entangled in Earth life. Major narrative arcs include the deepening of the mutual romantic confession between Hinata and Mariabell, struggles with her intense jealousy towards female classmates, and a significant arc involving the arrival of Mariabell’s mother, Miranellia, a powerful figure who travels across worlds to retrieve her daughter and disapproves of her relationship with a human. Another lighter arc involves Sakura forcing the couple to cosplay together to promote her doujinshi at a convention, blending the mundane world with fantasy chaos.