When Hiroaki introduces his classmate Nonoka to his older brother Ryuutarou, the two immediately clash in a flurry of insults. However, a moment of solitude leads to an unexpected sexual encounter, revealing that Nonoka is actually the mysterious "Sadako" who had been sending Ryuutarou love letters. Despite their hostile exterior, the two develop a secret, passionate bond that persists even after Ryuutarou moves out.
On Valentine's Day, Makoto accidentally catches his arrogant step-sister, Misaki, in his room, leading to a surprising confession of her long-hidden feelings for him. After sharing an intimate first time, the two establish a secret romantic connection, though Misaki continues to maintain her bossy and insulting exterior in public.
After Taiki and Chizuru begin dating, Taiki becomes distressed by rumors that Chizuru is involved with a teacher and attempts to break up with her for her own happiness. Chizuru quickly clears up the misunderstanding, revealing that the rumors were false and that she has loved Taiki since the moment they met. After reaffirming their feelings, the couple shares an intimate first experience and promises to support each other.