After her twin sister's death, Yoon-seo transfers into a new high school and is assigned the desk her sister once occupied. The boy beside her, Do-hyun, reacts as if a ghost sat down. What begins as a cruel coincidence becomes a chain of unsent letters, borrowed gestures, and one sealed message that suggests Yoon-ah understood more than anyone wanted her to.
At the Goldmoss Post-house on the imperial Salt Road, letters move by magic-stamped relay, and a ghost-stamped envelope can arrive before it was sent. Pae Soyeon, a broke backwater clerk supporting her siblings, knows the relay clauses better than anyone: she cannot refuse ghost-mail, she cannot answer it, and if she leaves her post her brother's debt-bond is forfeit. Then a letter arrives in her own handwriting, dated four months in the future: Stop reading the magistrate's mail. Each week, future-Soyeon writes again, angrier and more specific, naming crimes present-Soyeon has not yet committed, men she has not yet met, and children who have not yet died. As Soyeon investigates the magistrate's son, the courteous watch captain, and her own forged stamps on a caravan manifest, she realizes someone has been using her clerkship to launder atrocities through the relay. But the woman who already solved the mystery has chosen a simpler fix: come back on the new moon and kill herself before she becomes herself.
Yoon Seol works for the Night Post, a courier service that delivers letters the living cannot. When a parcel addressed to Kim Muwon—lighthouse keeper, died ten years ago—appears on her route, the red string on her wrist refuses to let her quit. At Cape Yeongdo she finds the keeper alive, the lighthouse lit, and a sealed letter that knows her name. To finish the delivery, Seol must open the one door she has spent her life avoiding: the night her father drowned.