We begin with Natsuo and Hina going on a “date” to Kamakura. The teahouse lady mistakes them for a couple. They see the sights and have a lot of fun; their chemistry is unassailable. Then they head for the beach, and Natsuo brings up Hina’s “child” comment from earlier.
Hina admits she was just trying to say the thing that would hurt Natsuo most, since she was already in a relationship with Shuu and she and Natsuo were now siblings. Then she tells him how she met Shuu: like Natsuo with Hina, he was her teacher and first love.
When her friends shunned her for being too cute and flirty, he was her only friend. When they met up by chance years later, he was wearing a ring, but she couldn’t turn him or her happiness down when he said they should get together.
When Natsuo hands her his newly-completed novel, whose heroine is modeled after her, he confesses he’s been in love with her for a long time. Hina’s reaction follows the general pattern of their incident in her bedroom: she draws closer, taking his hand, and proposing they go out together, keeping it a secret from their family and everyone else.
But then, as when she basically teased Natsuo’s lack of experience, Hina brings the hammer down, taking Natsuo a bit too far into the surf to make a point: for them to be together when they’re family will most certainly backfire stupendously. She likens it to double suicide, be it socially or literally.
Natsuo knows it’s not possible, but merely asks they stay in the surf a bit longer so he can hug her and cry it out. That night after they get back home, Hina reads the whole book in one night, and it brings her to tears. Through the pages she can probably feel Natsuo’s longing, because it’s exactly how she felt with Shuu. She can hardly blame him for something when she knows full well we aren’t in control of who we fall for.
Things seem to return to a friendly sibling relationship between Natsuo and Hina, but Rui’s crush on Natsuo continues, culminating in her visiting Natsuo’s room one night. She notes that on the day they met (and did it), they never actually kissed. She wants to try doing so now.
Despite things being cordial with Hina, Natsuo is still a wreck, and it’s at least partially his desire to prove Hina wrong about something like them being utterly impossible that leads him to acquiesce to Rui’s request. To hear Rui so earnestly describe how calm and at rest she felt while kissing him, well…it certainly complicates matters for young Natsuo.
At this point Miu seems to be the best bet for him in terms of romantic interests who aren’t related to him, while Momo would obviously welcome his company anytime. Still, with Rui stating her intentions to keep continue their kissing sessions on the downlow, it’s clear that it’s not going to be as easy as going out with Miu or Momo.
After spending the evening with Natsuo and Rui tutoring them for their upcoming exams, Rui brings up Ashihara and his apparent rapport with her, intriguing Hina. Later that night, while perhaps going to visit Rui’s room, Natsuo sees that Hina’s door is open.
Before he can knock, he hears a…a noise, and when he peeks through the crack in the door, he sees Hina pleasuring herself, letting out Shuu’s name when she finishes. I’m not sure why she didn’t completely close the door, but never mind; the deeply private moment Natuso witnessed can’t be unseen or unheard.
As disturbing as it was to see Natsuo linger by the door as long as he did, he saw in Hina what she sensed in him: an unbearable longing for the one they love. She may have broken things off with Shuu but she’s most definitely not over him. No doubt that will evoke some guilt in Natsuo, who, along with Rui, gave Hina such a harsh “him or us” ultimatum.