Lag's Dingo is the 4th overall episode of Tegami Bachi.
Lag and Niche reach the town they were looking for and find the address Niche is to be delivered to: a freak circus named "Love Someone Down". They discover that the animals used to perform in the circus have been mistreated. Niche is handed over to the circus ringleader and she bids farewell to Lag.
Before leaving for Yuusari, Lag forgets to have the receiver sign the delivery receipt and heads back. After obtaining information on the circus's newest exhibit, which is actually Niche, Lag rushes back just in time to see Niche destroying the circus and releasing all the animals. He goes after her into Broccoli Forest, which is gaichuu territory.
Lag finds Niche with a gaichuu, but she handles herself easily and cuts the creature's head off. The gaichuu continues its attack and injures Niche. This causes Lag to fire a shindan into the gaichuu, destroying it, while Niche sees Lag's memories. Niche decides to become Lag's Dingo, while an escaped animal from the circus, whom she calls Steak, goes with them.