Tegami Bachi Reverse is the second season to Tegami Bachi (discussed here) and continues exactly where the first season ends with Lag Seeing's cliff-hanger encounter with Gauche Suede/Noir. Whereas the first season focused on character development and plot background, the second season has more action and is a better story, so is actually more enjoyable than first season overall.
The main plot in season two concerns Reverse, an organization introduced in season one that is plotting to overthrow the government of Amberground. As the story progresses, it becomes obvious that Reverse's grievances are justified (sinister stuff is definitely happening in the capital) but their actions are fuelled purely by hatred, which is never a basis for good judgement so it's hard to understand how their plans would help anyone.
Reverse's members are all the result of failed experiments conducted by the capital, which caused them to "lose their heart". Some experiments even produced human-animal hybrids. It's easy to see why they might hate the capital, which turns them into misguided rebels rather than stereotypical bad guys.
A minor sub-plot explains Niche's origin as a "child of Maka" in great detail, complete with an actual Maka. The best part is Niche becomes smarter and gets an awesome power up after learning about her past, turning her into a seriously powerful dingo.
The major sub-plot explains why Gauche Suede the postman turned into the Noir the marauder, but tantalizingly, omits details of exactly how he "lost his heart" in the capital. However, Noir's story is more concerned with how he finds, with Lag's help, another purpose in life, beyond his simple hatred towards the capital. Although "losing one's heart" is never actually explained, the effect is that you lose your personality and most of your memories, leaving you a shadow of your former self.
The second season leaves a boat load more mysteries than the first season did. What exactly is going on in the capital? Something is happening there. What exactly is the artificial sun? It's definitely not a mechanical construct, that's for sure. Who or what is Lag Seeing really? It's clear from the final episode that he is much more than he appears to be.
I don't mind having some mysteries in a story, but this is bit too much, which suggests there may be a third season. However, with the main plot neatly tied up in a satisfactory conclusion, the producers have obviously not committed to continuing the story.