The story takes place in the land of Amber Ground, a place of perpetual night only partially illuminated by an artificial sun. Lag Seeing works as a "Letter Bee" (delivery boy) at the "Bee Hive" (a delivery service), with his Dingo, Niche, and her "pet", Steak, traveling with him. He is entrusted with the "hearts" of the everyone in Amber Ground to deliver their packages. 
(Source: Wikipedia) 
Note: The pilot episode of Tegami Bachi shown during the Jump Super Anime Tour in the Fall of 2008, the story was later incorporated into episode 17 of the first season.