Shinichiro Morisaki runs his late mother's Taisho-era café alongside his younger sister Kotomi. Their once-respected scientist father faced social ostracism and vanished after becoming consumed by android research, leaving the siblings impoverished. Shinichiro's life shifts dramatically when an android girl, Arisa, plummets through the café's ceiling, resulting in his registration as her master. Arisa assists with daily life, but her inherent clumsiness and ineptitude at maid duties complicate their routine; Shinichiro also develops a sexual relationship with her, where she serves as his sex slave. This relative peace shatters with the arrival of a girl searching for "Fire Bee," leading to the revelation of Arisa's true identity. Shinichiro discovers Arisa is the weaponized android "Fire Bee," the very project his father was developing before disappearing. This discovery attracts dangerous military attention, forcing Shinichiro to shield Arisa from external threats determined to reclaim her at any cost.