In a walled city perpetually at war, everyone's lives and livelihood depend upon maintaining and firing the enormous cannons that make up most of the city. Nearly every building in the city is equipped with a cannon of varying size, able to fire huge artillery shells over the city walls. Though the story is centered around a young boy and his father, who works as a lowly cannon-loader, the film is dedicated to the lives of the anonymous citizens of the city who slave to fuel and maintain this parody of the twentieth century war machine.

During the course of the film, the city is surrounded in clouds of smoke and dust and the mobile "enemy city" is never shown despite continuous reports of great success, leading the viewer to speculate if there really is an enemy at all, or if the walled city is simply firing into the clouds to perpetuate a war that has become its entire means of economy. This theme is similar to that of George Orwell’s Nineteen Eighty-Four albeit with a surrealistic tone.

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.