"Tegami Bachi: Letter Bee" is set in the fictional world of Amberground, a land perpetually shrouded in darkness, illuminated only by an artificial sun. The story centers on the delivery of letters and packages by specialized couriers known as Letter Bees. These couriers traverse dangerous terrains, often encountering hostile creatures called Gaichuu, which are drawn to the emotional energy contained within the letters, known as "Heart."

The protagonist, Lag Seeing, is a young Letter Bee who aspires to deliver messages with unwavering dedication. Lag’s journey begins with his recruitment into the Beehive, the organization responsible for mail delivery, after being inspired by Gauche Suede, the Letter Bee who once delivered him to his aunt as a child. Lag’s unique ability lies in his artificial left eye, the "Spirit Amber," which allows him to see the Heart within letters and people. This ability becomes crucial in his missions and his quest to uncover the mysteries of Amberground.

Lag’s primary companion is his "dingo," a human partner who assists in deliveries and combat. His first dingo is Niche, a mysterious girl with superhuman strength and a sword-like tail, accompanied by her small, sentient creature, Steak. Together, they face numerous challenges, including encounters with rogue Letter Bees, Gaichuu, and the enigmatic organization Reverse, which seeks to exploit the power of Heart for its own purposes.

As the story progresses, Lag learns more about the origins of Amberground, the true nature of Heart, and the connection between his Spirit Amber and the artificial sun. The narrative delves into themes of memory, loss, and the emotional weight carried by letters, as Lag strives to deliver messages that often hold deep significance for their recipients. The series also explores the backstories of key characters, including Gauche Suede, whose memories were erased by Reverse, and his sister, Sylvette, who becomes a central figure in Lag’s journey.

The plot intertwines personal growth with larger mysteries, as Lag uncovers the truth about Reverse’s plans and the role of the Spirit Amber in maintaining the balance of Amberground. The series balances action, emotional depth, and world-building, creating a compelling narrative that explores the power of connection and the importance of delivering hope in a world shrouded in darkness.