This is one of two extra episodes included with the BokuBen limited edition volume 14 and volume 16. As such, it is not must-watch TV in terms of plot. Instead, thanks to the beach setting, it’s an opportunity for all of the fanservice. Nariyuki and Asumi have been taking various lovey-dovey selfies for the benefit of her father, but when he presses them to go to the beach lest he start to develop doubts they’re really dating, well…what else can they do?
The thing is, Nariyuki can’t swim, so when a wave knocks them off their dinghy, he has to grab Asumi, and in the confusion her bikini top washes away. She wears his wet shirt until they find it washed up on rocks, along with a second, bigger bikini top, but it’s stolen by a dolphin.
Obviously, that top belongs to Kirisu-sensei, who is at that same beach with her fellow teachers at a BBQ. She had retreated from the unpleasant social situation and, well, lost her bikini top. Nariyuki knows her top is gone for good, so stays with her until the changing area clears out. When she trips on a sea cucumber and freaks out, she leaps at him, and he smashes his glasses.
Now Nariyuki truly finds himself in the “unusual situation” his sister could detect with her brocon-spidey-sense—virtually blind and having to be led through the rocks by his topless teacher. They find a shop where the only remaining swimsuit choices are an micro bikini and a school swimsuit. Nariyuki goes for the coverage.
When the other teachers (who are dogs, BTW) spot Nariyuki and ask about Kirisu, he lies and says he hasn’t seen her. Kirisu explains her reluctance to large social events to her past as a solo competitor, viewing everyone as her enemy. Having both covered her (both literally and figuratively), Nariyuki briefly loses himself and comes out and says she’d be in big trouble were it not for him on numerous occasions.
He was trying to make a point on the benefits, and often necessity of occasionally relying on others, as she most certainly has with him, but it came out wrong. To his surprise, he doesn’t get socked for his comments; Kirisu agrees. In a way, he’s kind of a tutor for her, only on matters like this (it’s unlikely she’ll ever learn to clean properly).
Finally, the episode closes on Fumino, Uruka, Rizu and Sawako wearing swimsuits while they study, since they’re not able to go to the beach. Uh huh. Surrre, why not?!