Tegami Bachi takes place in the land of AmberGround, a place of perpetual night only partially illuminated by an artificial sun. Lag Seeing works as a Letter Bee at the government delivery service named the Bee Hive. He travels with his Dingo, Niche, and her "pet", Steak and is entrusted with the hearts of everyone in AmberGround as he delivers their packages.

(Source: Wikipedia)

Note: Volume 3 contains the special chapter "Tegami Bachi to Dingo" (Letter Bee and Dingo).