Lag, ayant entrevu le cœur de Gauche, apprend qu'il rêve d'être choisi pour le titre le plus élevé que les Abeilles qui travaillent dans la capitale peuvent obtenir, celui d'Abeille en chef, et qu'il y sera bientôt transféré, un premier pas vers ce rêve. Vers la fin de leur voyage, un jour avant d'atteindre leur destination, Lag et Gauche se disputent après que Lag a demandé à Gauche de l'emmener avec lui à la capitale pour qu'il puisse rechercher sa mère. En colère, Lag s'enfuit avec le shindanjuu de Gauche et se dépêche d'avancer seul, mais il s'égare dans le territoire des Gaichuu sans s'en rendre compte et tombe dans un piège.

Quelques années après avoir rencontré Gauche, Lag se rend au bureau de poste principal de Yuusari afin de passer l'entretien pour devenir un Letter Bee après avoir réussi l'examen préliminaire. Après avoir fait ses adieux aux personnes qu'il aime, il se rend à la gare avec un Letter Bee nommé Connor qui est venu le chercher. Mais lorsqu'il arrive, il découvre une petite fille qui se cache dans une petite fissure. Et en plus, c'est une lettre, tout comme Lag !

Lag a donné le nom de "Niche" à une fille qui était une lettre dont le bordereau de livraison était incomplet et n'a pas pu être livré, et a commencé à l'emmener à sa destination. Lag est enchanté par ses cheveux pendant qu'il l'escorte, et en chemin après l'avoir déposée, il apprend de quelques passants que sa destination était en fait un spectacle de monstres, faisant la publicité d'une nouvelle attraction : Un enfant qui a reçu le sang de la créature légendaire "Maka".

Lag, ainsi que Niche et Steak, un étrange animal qui accompagne Niche, atteignent la ville de Kyrie, où le pont Bifrost relie Yuusari. Ils y rencontrent Nelli, qui les guide jusqu'à une auberge pour y passer la nuit, et se porte volontaire pour monter le sac de Lag dans sa chambre, mais disparaît soudainement. Le but de Nelli est en fait d'obtenir le laissez-passer de Lag pour Yuusari.

Lag et Niche courent après Nelli pour récupérer leur laissez-passer, mais Nelli est découverte comme un imposteur et va être punie ! Lag et Niche s'approchent pour sauver Nelli, mais à ce moment-là, la lettre que feu Nello, le frère de Nelli, a écrite à Jiggy Pepper s'envole. Lag tend son bras de toutes ses forces pour sauver la lettre, et soudain un shindan s'envole de sa main !

Lag décide de faire de Niche son dingo officiel et se rend au bureau de poste de Yuusari, connu sous le nom de Bee-Hive. Lui et un autre garçon nommé Macky Gee commencent l'examen d'aptitude à la fonction d'Abeille, qui consiste à livrer une vraie lettre en toute sécurité à sa destination. Sachant qu'il s'agit d'une vraie lettre, Lag est encore plus déterminé.

Gauche n'est plus une Abeille. Lorsque Lag l'apprend par Zazie, la surveillante de l'examen des Abeilles, il est choqué et déprimé. Lag, Nice et Steak vont voir Sylvette, la sœur de Gauche, mais ils sont pris pour des agents de recouvrement et accueillis par une fille sévère en fauteuil roulant qui les menace d'un pistolet

Lag se rend une nouvelle fois chez Sylvette, seule dans sa maison, et son cœur est touché lorsqu'il la voit pleurer sur la véranda. Lag envoie un shindan de son propre corps dans le shindanjuu que Gauche a laissé derrière lui, le Nocturne Op. 20 Shindan Custom, et il apprend, avec Sylvette, ce qu'il y avait dans le cœur de Gauche.

On the morning of the first day of work for new Letter Bee, Lag Seeing, and his dingo Niche, the sub-master of the Bee-Hive, Aria Link, stops by Sylvette's house where Lag is now living. She shows him to a place known as the Hill of Prayer, where everything with Gauche had its beginnings.

Lag begins his daily life as a Letter Bee, and one day goes to pick up a letter from a young man named Vincent Alcott. He has been unsuccessful in fulfilling his dream of becoming an author and in order not to worry his mother back in his hometown, he writes letters filled with lies about his success. When Lag finds out about this, he worries whether such a "letter of lies" really holds any heart.

It's the "Holy Night" season, a festival from the northern area of Amberground. They say that two old men go around on the Holy Night delivering presents to children. In order to continue that tradition, Lag, along with Zazie, deliver presents to children that have been collected by charitable organizations. They meet a girl, Sonja Malmola, and granting her wish, they stay for dinner with her. Zazie helps with the shopping, but the way the villagers treat the girl is suspicious. Her parents were killed by gaichuu, and she lives all by herself.

One day a man named Promessa Prometas came to the Bee-Hive asking for a letter he sent back. But the letter had already been sent out on delivery, with the new Letter Bee Lag Seeing. When Promessa hears this he chases off after Lag. On the other hand, Lag is worried that his journey is going too smoothly, because the letter has a fixed delivery-date. At this rate he'll end up arriving at the destination early. Just then,Lag runs into a man named Galador...

Lag's friends Zazie and Connor recommend he go for a checkup with the medical team, after he arrives at work one day exhausted from his busy days as a Letter Bee. The head of the medical team is Dr. Thunderland Jr., a man with an odd taste for dissection, leading him to be called "The Corpse Doctor" by many out of fear. He takes an interest in Steak and captures him, because Steak is a species known as a Kapellmeister which was thought to be extinct. Lag and Niche hurry after the doctor to rescue their friend...

Lag and Niche are riding a carriage on the way back from a delivery, and the two other passengers are a passionate couple named Bonny and Moss. They've promised to marry each other, but Frank and his gang, who's proposal was rejected, are chasing after them. Niche, after hearing that eloping means getting to be together with the one you love, drives away Frank so Bonny and Moss can be happy together, but the real reason Frank was chasing them turns out to be something else...

Lag delivers a fan letter to the popular pianist Mathilda Rein from a passionate fan of hers named Joey, and learns that she always enjoys reading his letters. But by accident he finds out that those letters are actually written by a different person...

Lag and Niche are to deliver a small package to a town in Yodaka named Silencio, sent by the Bee-Hive master Largo Lloyd himself. The recipient is a woman who once worked as a Letter Bee, Elena Bran. However unlike a usual delivery, this time Lag is assigned with him a guide named Darwin...

One day, a trio calling themselves the "Letter Pigeons" barge into the Bee-Hive and challenge the Bees to a race, in order to make a name for themselves and start a letter delivery service besides the Letter Bees. Master Largo thinks it sounds like fun so Lag, Zazie, and Connor take on the Letter Pigeons in a fierce delivery battle!

Lag gets a fever. Sylvette and Niche are worried and try nursing him back to health, but Niche isn't used to it and makes mistake after mistake. And whether it's on an errand or shopping she keeps getting distracted as well. Sylvette takes care of them one after another, but Niche dislikes how Sylvette is always nagging her.

Zazie comes back after having lost important letters, and he's injured, too. As the rumor goes around that Zazie threw away the letters in order to escape from the gaichuu, another Letter Bee is to be dispatched from the Bee-Hive to recover the letters and finish the delivery. Lag jumps in to volunteer, but it seems that Zazie braved a dangerous route spotted with gaichuu territories.

A therapy relaxation room planned out by Thunderland Jr. has opened up in the Bee-Hive. Lag and Connor are curious and drag along Zazie to go check it out right away. The caretaker for the rest and relaxation room, Mana Jones, lost her sight in an accident, but she has an amazing sense of smell and is a plant specialist. Mana heard about Lag from Gauche in the past, and begins to tell him about what happened during the accident and why Gauche and Dr. Thunderland Jr. are the ones who enabled her to continue her work.

Gauche hurries to deliver a letter from Dr. Thunderland Jr. to Dr. Whitman, the foremost authority on vegetation and herbal research, asking him for a recommendation in order to prove the value of Mana Jones's research to the council. However Dr. Whitman lives far away, and there is not enough time to make it there and back within the given deadline. Even so, Gauche takes on the nearly impossible task...

Lag, Connor, and Niche encounter a married couple of Hunt and Sarah calling themselves "ones who were unable to become Spirit" in the town of Honey Waters after accepting a delivery of a letter there by Thunderland Jr, and Sarah seems to possess a black spirit amber like the one Gauche Suede had. The two of them have taken up the mantel of the anti-government organization "Reverse" and attack Lag and the others. Niche and Connor are captured, while Lag manages to barely escape, when a mysterious girl appears before him.

The larger number of letters that Ann Grado, the girl Lag met, brought out, drew a gaichuu to the town. Lag and Connor worked together to defeat it, but the people in the town got in their way and things were looking bad! Just then, Zazie shows up on his way back from a delivery. The three of them team up and corner the gaichuu, but...

Ann and Hunt are grabbed by the gaichuu. While Zazie and Connor struggle desperately to attack it, Lag lets loose a shindan that finally catches the gaichuu. His strength exhausted, Hunt falls toward the ground. Hunt, hit with Lag's shindan, projects an image of Sarah and Hunt's cruel past as Gauche and Roda fight to protect the two from a gaichuu.