A student named Ayumu, whose mind is always filled with lewd thoughts but who is an unpopular loner. One day, he sees Ranka, a black gyaru known for being promiscuous and living in the same apartment building, masturbating while giving head to a man on the staircase. He can't help but record it with his phone, but it seems he got caught. Later, he is ambushed on the staircase and taken to Ranka's room...
