Chu-Bra!!

Season 1, Episode 7: Swaying Hearts of Men

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Hiroki's newfound knowledge of underwear is affecting both his concentration and his reputation. Nayu gets a mysterious 'love' letter in her shoebox. Hiroki gets annoyed and decides to quit the underwear society. On his way home, he runs into Keigo, who tells him about Nayu's efforts to make the society more male-friendly. Nayu's confessor had apparently put his letter in the wrong shoe box, and Hiroki apologises to Nayu and gives the society another shot.