In a mystical land called Amberground are Letter Bees-national letter carriers whose job is to deliver people's “hearts” across the land plagued with dangerous beasts. Letters, parcels, humans, animals-they carry anything that embodies senders' feelings or hearts that they wish to convey to their receivers.
Lag Seeing has once been delivered by a Letter Bee named Goos when he was little. The memory of Goos risking his life to carry Lag left Lag with aspiration to become a Letter Bee himself. A few years later, Lag starts his career as a novice Letter Bee with Niche, a bodyguard girl with legendary creature's blood in her veins. In the course of his profession, Lag learns that his hero Goos is no longer registered as a Letter Bee. Lag swears to search Goos out and find out the secret behind his disappearance.