Tengoro-hen 
Due to a mix-up, Shizumori's underwear ends up being put into the wrong locker by a classmate after P.E. Shizumori now has to spend half the school day without wearing any underwear until she is finally able to return to the locker room. Once there, she's surprised by Ozaki smelling her underwear. He gives her back her bra, but throws the rest on the ground. When Shizumori's leans down to grab it, he makes a candid shot of her wearing no underwear. Armed with photo-evidence of her being an "exhibitionist" he blackmails Shizumori into having sex with him. 
(Source: hentaistream)