One minute Fujii Natsuo is at an innocuous mixer, the next he’s sneaking out with the least enthusiastic of the girls, Tachibana Rui who asks him for a favor. She wants to have sex, so she knows what it’s like and so it won’t feel like people are talking down to her.
She correctly assessed that Natsuo was also a virgin, and so she wouldn’t be taken advantage of or anything else by a more experienced guy. They get undressed, do the deed, get dressed, and part ways. There was never supposed to be any passion or emotion of any kind, so Rui tells him that if they ever meet again, they’re strangers.
As Natsuo confides to his dorkier best friend (who make it a point to de-dorkify Natsuo before high school so at least one of them would have a chance, socially speaking), the experience doesn’t quite sit that well with him, the more time that passes after The Act.
For one thing, there’s someone he likes—a pretty young teacher at school—and while it’s most likely to remain unrequited (at least as long as he’s a minor and her student), a part of him laments that his first time was so…impersonal.
But if you think the chain of events Natsuo has experienced to this point was sudden, you ain’t seen nothin’ yet! One afternoon his father announces he’s going to remarry, ten years after Natsuo’s mother passed away. And oh yeah, she’s coming over right now, with her two daughters…who just happen to be Natsuo’s teacher Hina and Rui, with whom he had sex. OH, SNAP!
Natsuo faints from the shock, but when he comes to, Hina and Rui’s mother warmly introduces herself, and when asked, tells him why she likes him. She and his father really are a good match and both seem happier than any of the kids have ever seen them. It’s for that reason that Rui, who waits for Natsuo outside the toilet, is not going to get in the way of her mom’s happiness just for “some stupid reason” (read: their having sex that one time).
Natsuo gives his dad his blessing as well, and things only accelerate from there: his dad buys a new house big enough for the newly-combined family, and adding to Natsuo’s apparent woes. Hina insists on a dropping of formalities in the house, while Rui insists that Natsuo forget they ever did it, as it’s clearly still bothering him. Naturally, that’s not so easily done; Natsuo can’t get the images of making love to Rui out of his head.
Still, This Is All Happening and everyone has to make the best of it. For Hina’s part, she forgets she’s no longer in an all-women’s household and comes out of the bath half-naked, only to put on a loose tank and short shorts before drinking herself to sleep with Asahi Super Dry (not out of depression; she just likes to drink).
As she dozes on the couch, with everyone else in bed, Natsuo decides he’ll close the book on his one-sided crush on his teacher-now-stepsister once and for all…with a first and last kiss. But before his lips meet hers, Rui enters the room, and her expression isn’t one of total contempt, but something more like…a “how could you?” face.
Your enjoyment of a show like this (or Koi to Uso, or Kuzu no Honkai) will depend entirely on your stomach for love triangles (and other polygons) as well as your ability to swallow a premise this specific and weird. To its credit, because of the central reason for the new arrangement—his dad and their mom are truly in love and deserve happiness—helps temper the ridiculousness.
Because the dynamic between Hina and Natsuo and Rui and Natsuo are so different, it will be very interesting to see how the three navigate their new normal.