Yoshifumi Nitta, a mid-level yakuza, finds his life forever changed when a strange capsule appears and rams into his head. Though he believes the curious event to be a dream, he finds the capsule still there the next morning; from within it emerges a young girl. She remembers nothing but her name, Hina, and uses psychokinesis to coerce Yoshifumi into buying her clothes and toys. Unable to get rid of Hina, Yoshifumi reluctantly becomes her guardian.

The pair's peculiar life is just beginning. As Hina lazes around the house, Yoshifumi quickly rises through the ranks of the yakuza with the help of her supernatural abilities. Hinamatsuri follows the comedic duo as Hina drags both new friends and old acquaintances into her antics, while Yoshifumi juggles between taking care of Hina, hiding her powers, and managing the yakuza business.