You are Tominaga Yahiko, a dead-end salaryman in the planning department of a travel agency. All of your tour plans have been rejected recently, and every time your superior bullies and power harasses you. You come home exhausted every night, and the stress has even caused erectile dysfunction. But one night, something changes. As usual, you return to your rundown apartment and crawl into bed to sleep. You wake up in the middle of the night to find an astonishingly beautiful girl leaning her face close to your crotch. "You've got a pretty good thing going on down here ♪" Red eyes that seem to suck you in. Black horns growing out of her head. And most of all, a pair of magnificently large breasts. As you are stunned by the strange beauty in front of you, the mysterious girl — a self-proclaimed demon — reaches her tongue towards your member. And then, with her demonic technique, you are cured of your erectile dysfunction and, in a fit of pleasure, come for the first time in a long time. Unlike you, however, the demon seems dissatisfied. "Your dick should be capable of much more than this. I'm going to draw it out from you!" With this unilateral declaration, the demon girl starts living with you. The demon's target is the high-quality magical power contained in your semen. She plans to use it to revive the sealed Maou. During such days of being drained by the demon, a female angel appears before you. Turns out that the angel is actually the superior who was bullying you. "Rather than letting the demon take your magical power, I'll milk your semen myself!" The angel tries to prevent the demon from retrieving the magical power contained in your semen by milking it first. And so the battle for your magical semen, which is necessary for the Maou's resurrection, begins.