Light and Blue Night Fantasy is the special episode (Pilot) of Tegami Bachi - not part of the original canon. It eventually became part of the story in Letter Bee and Dingo.
Lag Seeing finishes a delivery and returns to the bee hive to receive his next assignment. The chief explains that it's a small package for Elena Bran, who once worked as a bee, and a letter to Bart Bran, her younger brother in Yodaka. The letter is from Largo Lloyd, the chief.
Aria Link tells Lag that there will be a guide around Lake Olympia known as Darwin. He finds out that Darwin is a veteran. Lag Seeing and the others follow Darwin to Bifrost. When Aria and the chief are in a conversation, Aria asks the chief why he hid from Lag that Darwin was Elena Bran's Dingo when she's on duty and the real package to Elena is Darwin.
Darwin fainted in the journey to George's Little Village. Lag sent him to a man's house, who was Reverend Weller. When Lag looked at Gauche Suede's shindanjuu, he started thinking about the past with Gauche Suede. The man called Lag to be careful of the Dacquiose Cliff, because Elena Bran was killed by a Gaichuu 10 years ago there. Lag decided to leave Darwin there because he's still weak. But Darwin was awake and forced himself to go with them when he heard about Elena Bran.
At the Dacquoise Cliff near Silencio, Niche finds no path because there was a landslide. Gaichuu Absinthe attacks, and when Lag shoots a Shindan, he sees Darwin's heart. Elena had gone for a delivery and left Darwin at the Olympia Lake because he was hurt, and called him to wait for her but she died. Largo Lloyd (when the chief was young and still only a Bee) told Darwin not to wait, but Darwin didn't care about his word and kept on waiting.
After Lag followed Darwin who passed through the cave to Silencio, he saw Elena's grave. Darwin laid beside Elena's grave and died. Lag saw Elena's heart. Largo Lloyd and Elena were good friends before she died, and she asked about his charm because it's nice. He told her that it's a rare item, but there is another matching pair.
When the chief found it, he wanted it delivered to Elena. Lag sent the small package (a pair of charms) and the letter to Bart, Elena's brother. After that, Bart Bran made a grave for Darwin beside Elena's grave and placed the charms on both of the graves of Elena and Darwin.