"Tegami Bachi" is set in the fictional world of Amberground, a land perpetually shrouded in darkness due to an artificial sun that provides only minimal light. The story revolves around the delivery service known as the Letter Bee, where messengers, called Letter Bees, transport letters and packages across dangerous terrains inhabited by monstrous creatures known as Gaichuu. These Gaichuu are drawn to the "heart" energy contained within the letters, making the job of a Letter Bee perilous.

The protagonist, Lag Seeing, is a young and determined Letter Bee who carries a deep sense of purpose. Lag’s journey begins with his aspiration to deliver letters that carry the emotions and "heart" of the sender to their recipients. His own heart was once taken by a mysterious figure, Gauche Suede, who was a Letter Bee and a mentor to Lag. Gauche’s disappearance and subsequent memory loss serve as a central mystery driving Lag’s motivations. Lag is accompanied by his Dingo, a human companion named Niche, and her shape-shifting pet, Steak. Niche provides both physical protection and emotional support, while Steak aids in combat against Gaichuu.

As Lag travels through Amberground, he encounters various characters, each with their own struggles and connections to the Letter Bee service. The narrative explores themes of memory, loss, and the power of human connection, as Lag delivers letters that often reveal poignant stories about the people of Amberground. The series delves into the hierarchical structure of Amberground, where the capital city of Akatsuki is the only place with access to the artificial sun, creating a stark divide between the privileged and the impoverished.

The plot thickens as Lag uncovers secrets about the government organization known as Reverse, which is involved in sinister experiments and the manipulation of "heart" energy. Reverse’s activities are tied to the disappearance of Gauche, who is revealed to have been brainwashed and turned into an agent of Reverse under the name Noir. Lag’s determination to save Gauche and restore his memories becomes a focal point of the story, intertwining personal stakes with broader societal conflicts.

Throughout the series, the bond between Lag and Niche deepens, and their partnership becomes a symbol of resilience and hope in a world dominated by darkness and despair. The narrative balances episodic storytelling with an overarching plot, blending action, mystery, and emotional depth. "Tegami Bachi" presents a unique blend of fantasy and steampunk elements, creating a distinctive atmosphere that complements its exploration of human emotions and the enduring power of communication.