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.