Yui Kasugano is staying late at the office, working alone. Suddenly, she realizes she hasn't cleaned her navel recently. In the empty office, she begins cleaning her navel, and the accumulated black navel lint falls out. Sensing the pleasure it brings, Yui starts using her navel for masturbation. With her slender figure, she relaxes in the room with her navel exposed. When she feels hungry and rubs her stomach, her hand naturally touches her navel. Realizing how good it feels, she finally indulges in navel masturbation.