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...