One day Lag Seeing finds himself shipped. His mother abducted by someone, he's now just a package sent by her to be delivered to a friend's house. Gauche Suede is a Letter Bee, a member of the Amberground national postal service, who is tasked with delivering this "letter," together with his partner Roda. The still young Lag rebels at Gauche's seemingly emotionless focus on his work, his heart still reeling from being torn from his mother, but as they head into a valley they are attacked by a ferocious creature known as a Gaichuu.

Lag, having glimpsed into Gauche's heart, comes to know about his dream to be chosen for the highest title the Bee's that work in the capital can obtain, Head Bee, and that he will soon be transferred there, a first step toward that dream. Nearing the end of their journey, one day before reaching their destination Lag and Gauche get in a fight after Lag asks Gauche to take him with him to the capital so he can search for his mother. Angry, Lag takes off with Gauche's shindanjuu and hurries along by himself, but wanders into Gaichuu territory without realizing it and falls into a trap.

Some years after he met Gauche, Lag heads to the main post office in Yuusari to take the interview to become a Letter Bee after passing the preliminary examination. After saying all his goodbyes to the people he loves, he heads to the train station with a Letter Bee named Connor that came to pick him up. But when he arrives, he discovers a small girl hiding herself in a small crevice. And what's more, she's a letter, just like Lag was!

Lag gave the name "Niche" to a girl that was a letter whose delivery slip was incomplete and couldn't be delivered, and started taking her to her destination. Lag is enchanted with her hair while he escorts her, and on his way after dropping her off, he learns from some passersby that her destination was actually a freak-show, advertising a new attraction: A child who received the blood of the legendary creature "Maka."

Lag, as well as Niche and Steak, a strange animal that tags along with Niche, reaches the town of Kyrie where the bridge Bifrost connects to Yuusari. There they meet Nelli, who guides them to an inn to stay the night, and volunteers to take Lag's bag up to his room, but then suddenly disappears. Nelli's aim is actually to get Lag's crossing pass into Yuusari.

Lag and Niche run after Nelli to get their crossing pass back, however Nelli is found out as an impostor and is going to be punished! Lag and Niche come up to save Nelli, but just then the letter Nelli's late brother Nello wrote to Jiggy Pepper flies away. Lag stretches out his arm with all his might trying to save the letter, and suddenly a shindan flies out from his hand!

Lag decides to make Niche his dingo officially, and reaches the Yuusari Head Post Office, known as the Bee-Hive. He and another boy named Macky Gee begin the Bee suitability interview examination, which consists of delivering one real letter safely to its destination. Knowing he is delivering a real letter, Lag's determination is strengthened yet again.

Gauche is no longer a Bee. When Lag heard this from the Bee examination proctor Zazie, he's shocked and depressed. Lag, Nice, and Steak go to see Sylvette, Gauche's sister, but are mistaken for debt collectors and met by a stern, wheelchair bound girl who points a gun at them.

Lag goes once again to see Sylvette, alone in her house, and his heart is struck when he sees her crying on the veranda. Lag shoots a shindan from his own body into the shindanjuu Gauche left behind, the Nocturne Op. 20 Shindan Custom, and he, along with Sylvette, learn what was in Gauche's heart. The memories in Gauche's shindanjuu are...

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.

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.