The Story of Doing Something Bad With Hikari

On her way to school, Hikari races against her homeroom teacher, Mato Katori, who she discovers to be a fellow scuba diver. Later that day, Hikari drags Futaba to the school's Diving Club, where they try on some drysuits and take a dip in the pool. Moved by Hikari's high spirits, Futaba decides to join her in diving.