Based on eroge (erotic game) Oni Chichi ~Manamusume Kyousei Hatsujou~ by Blue Gale.
Marina and Airi's mother divorced few years ago and got remarried recently to a young man. While Marina accepts her new stepfather, Airi detests him. Little do they know that really he's a pervert that sniffs their underwear and wants to make them his own by making a love drug that will turn Marina and Airi into a sex-hungry sluts. When Airi comes home and notices her schoolmate Sana giving him a blowjob, he sprays the drug on her. Now he will do with them whatever he pleases.