The Letter Bees are tasked by the government with going after the
Cabernet, which although weakened by the attack that Lag and Gauche made
is still considered a serious threat. With not enough personnel left
at the Hive as a result to carry out normal deliveries, Aria's left
dealing with the rounds herself, and one of her deliveries sees her
carrying a letter to one Houdai Franklin, who was on the airship that
was nearby Amberground's artificial sun during the Day of Flicker.
Niche tags along as her dingo, but the new duo has a tough time working
Aria is a complete klutz. While she's the model of grace and poise around the Hive, no sooner has she struggled into her delivery uniform (and she's clearly grown a bit since the last time she had to wear it), she turns into a top-grade clumsy girl, tripping and falling over any obstacle. Which does make you wonder how she ever became a Bee in the first place, given that successfully making deliveries is a key part of the test that all new Bees have to take (one of a few little inconsistencies that the picky could point at in this episode). I have to say, though, that there's something about clumsy anime girls that definitely adds to their adorableness score, and this episode moves Aria from the 'cold and aloof' category to somewhere much closer to 'daaawwww'.
Lag's out of action from exhaustion and, when he hears of Aria's legendary clumsiness, he decides to send Niche along the help her out - but the two really don't get along. Niche is used to Lag letting her have her own way, for the most part, while Aria's own experiences with her own dingo (a big hairball of a dog by the name of Bolt, who's now too old and infirm to be a working dog) means that there's a huge gap between what she expects an dingo to be, and how Niche behaves in the role, that leads to a great deal of friction between them.