Hinamatsuri centers on Yoshifumi Nitta, a yakuza member whose rare moment of satisfaction over acquiring a prized vase is shattered when a strange capsule drops into his life and lands on his head. Inside is Hina, a blue-haired girl who reveals almost nothing about herself except her name and her immense psychic power. Because that power has to be used regularly or it will erupt in an explosion, Nitta is forced into the role of her reluctant guardian. What starts as a bizarre inconvenience soon turns into a practical partnership: Nitta sends Hina to help with a construction deal, the job goes smoothly enough to make him think she might be useful to his criminal business, and when a rival yakuza group covertly attacks his boss and Nitta's own colleagues decide to place the blame on him, Hina intervenes during the retaliation and wipes out the entire enemy group.
From there, Nitta realizes that Hina can be an invaluable asset as long as she never turns those powers on him, and their strange everyday life settles into a chaotic mix of violence, dependence, and deadpan comedy. The series expands that premise through a wider cast that includes Hitomi Mishima, Anzu, and Mao, broadening the story beyond the initial yakuza conflict while keeping Hina and Nitta at its center. Across the franchise, psychic chaos and mundane daily life collide in a comedy built on their uneasy household and the trouble that follows Hina wherever she goes.