Yuka and Kaoru are childhood friends, Kaoru is timid and Yuka is pretty and popular. Yuka grows out of Kaoru and doesn’t like him that much anymore. The day of the gender test comes in middle school where the find out if they’re alpha/omega/beta. Yuka turns out to be an omega and gets attacked by his group of friends and Kaoru tries to defend him. He ends up getting raped by Kaoru to save him from the others and he bites him and they become a pair. They’re together for years just as awkward friends still who just have sex during the heats. But then develop stronger feelings for each other.
Taiga, one of the boys who attacked and tried to rape Yuka, is now an adult and has an omega partner, Rin. Rin is heavily traumatized and is stuck at Taiga’s house because he’s broke. Rin overdoses on omega suppressants for months because he hates Taiga having sex with him and thinks his fated pair is actually Kaoru.
Yuka is supposedly actually Taiga’s fated pair, and vice versa with Rin and Taoru.
Taiga finally realizes that he actually loves Rin and they become a pair. Then Rin starts to lose his memory as a side effect of overdosing, and he eventually forgets who Taiga is. But Taiga is so in love that he reminds him every day.
In the end Yuka and Kaoru have a child, and Kaoru’s work in the pharmaceutical field helps develop more precautions for omega suppressants for the future.
Shounen No Kyoukai