Set in a fictional land illuminated by an artificial sun, it follows Lag Seeing, a Letter Bee with the ability to see the memories of people and items, and his personal dingo Niche on their journeys across AmberGround.

In a dark land of near-perpetual night, only a single artificial sun illuminates a single city, and communication between the rest of the world is made possible only by the Letter Bees - elite couriers who must fight against monsters known as Gaichuu in order to make their deliveries. Having been saved by a Letter Bee when his mother was kidnapped years ago, Lag Seeing has finally gained employment at the Bee-hive, but he's quickly learning that the dangerous job has become even more lethal, with renegade humans now targeting the Letter Bees as well. Fortunately, Lag's been blessed with the ability to see into the memories of others, and every Bee is also supposed to find a "Dingo," a partner to watch their back. However, when Lag finally finds his Dingo, she's not at all what he expected. In fact, she may end up being the girl who kicks over the entire hornet's nest. But no matter what the stings of fate, Lag isn't going to let anything clip his wings.—Sentai Filmworks

With his mother taken away from him and having lost everything, Lag Seeing is now a letter whose delivery has been assigned to Gauche Suede, a Letter Bee. Despite their troubling start, the two of them become friends, leading Lag to realize what his aim in life is: to deliver people's most important feelings in the form of letters, just as Gauche has done.—MAL Rewrite

Gauche Suede is on his last delivery before a big promotion. In the outskirts of Yodaka, the darkest area of Amberground, Gauche is surprised to find that the package is a young boy named Lag Seeing. Lag had been traumatized by his mother's abduction and is due to be delivered to his aunt. In this remote area rife with Gaichuu, Lag and Gauche face a dangerous journey that inspires Lag to become a Letter Bee.—Crunchyroll

Apparently abandoned by his mother, orphan Lag Seeing is taken as a "letter" by "Letter Bee" Gauche Suede to a town where his aunt raises him. His affection and admiration for Gauche give Lag the ambition to become a Letter Bee himself, carrying mail through the dangerous wasteland between towns on his world. When he becomes a teenager, he makes the trip to the capital to take the examination to become a trainee Letter Bee. En route, he meets the strange, powerful girl-like creature Niche, who becomes his "dingo," a Letter Bee's travelling companion. Lag yearns to find his mother, and discover why she abandoned him, if she did.—Anime News Network