Call it the letter beetwo separate anime television series, entitled Tegami Bachi and Tegami Bachi REVERSE respectively.The story takes place in AmberGround, a land of perpetual night only partly illuminated by an artificial sun. Lag Seeing is a newly minted delivery boy, called a "Letter Bee", who worked at the Bee Hive delivery service with his Dingo, Niche, and her "pet", Steak, travelling with him. As a Letter Bee, Lag's job is to deliver letters and packages from town to town while avoiding AmberGround's deadliest hazard—Gaichuu, giant armoured insects who attempt to feed off the "heart" that reside within these letters and packages.
When Lag was very young, his mother was kidnapped by men from AmberGround's capital of Akatsuki. He was sent as a "delivery" to his aunt thanks to Gauche Suede, who Lag started to idolize and was inspired to become a Letter Bee. But as Lag becomes a Letter Bee, he is informed that Gauche has disappeared; at the same time, a resistance movement called "Reverse" begins stealing letters from travelling Bees.
Lag later encounters Gauche, who appears to have no memory of his past, and is now devoted to Reverse's cause. In desperation, Lag vainly tries to make Gauche remember. At the same time, Gauche awakens a giant flying Gaichuu that is headed for the town Yuusari, the location of the Bee Hive; and later Akatsuki to destroy AmberGround's man-made sun.