Tegami Bachi (or Letter Bees) are couriers that deliver important letters and packages (which carry a part of the sender's heart) throughout the land of eternal dusk, Amber Ground. Gaichu are insectoid monsters that feed on the hearts (feelings) of humans.

The story revolves around Lag Seeing, a young boy who was born on The Day of Flicker, 12 years ago when the man-made sun went momentarily dark for unknown reasons. The manga follows his ambition to become a Letter Bee and how he follows the path of his idol Gauche Suede who went missing...

Together with his trusty dingo, "Niche", he will make new friends, face many perils, feel happiness, sadness, pain, come face to face with the darkest secrets of Amber Ground and the truth about himself.