An very good prequel to the legendary OVA. Not as great as an original story, but still a nice treat for the fans.
I think it would be useful to mention that the chronological timeline of the arcs are mixed. I recommend watching them in chronological order.
I'd like to briefly talk about each and every arc.
"Spiral Maze"
This arc is about Yang's early years, the backstory of El Facil, his subsequent appointment and the investigation of a many-year-old case involving Admiral Bruce Ashby, a character that was briefly mentioned in the original series.
Overall, we've got a pretty good detective story mixed with some new worldbuilding and further exploration of Yang's character.
"Valley of White Silver"
Reinhard and Kircheis's first assignment after the academy was to the distant ice planet Kapche-lanka. This is an action-packed survival story of two guys on a desolate ice planet. Pretty cool.
"Mutineer"
Reinhard and Kircheis were promoted and transferred to the Imperial Navy, where they were appointed chief navigator and security officer aboard the battleship Hameln II and encountered a critical situation during one of their patrols. One of the best arcs in the gaiden, in my opinion, perfectly showcasing Reinhard's talents. It's very intense and keeps you on the edge of your seat throughout the entire second half of the arc.
"The Duelist"
Covers the events where Reinhard became involved in imperial power politics as he volunteered to duel on behalf of Viscountess Dorothea von Schaffhausen, a friend of his sister Annerose von Grunewald, to settle a dispute over mining rights in the mines with Christoph von Herxheimer. But the higher nobility do not like his desire and the duel will not be as simple as it seems. A pretty good action-packed arc. Not my personal favorite, but still good.
"The Retriever"
This arc covers a secret mission assigned to Reinhard von Musel by military intelligence to infiltrate Alliance territory with only a cruiser under his command, to apprehend a defecting nobleman and recover a top secret military device that the defector stole before escaping. But is this only necessary for intelligence, since the defector is none other than Christoph von Herxheimer, the guy from the previous arc.
It's pretty good. Nothing special to say about this arc, but I can't say that I'm disappointed either.
"Dreams of the morning, songs of the night"
Reinhard and Kircheis are transferred to military police training on Odin, during a time of temporary peace at the front, where they are assigned to investigate a murder case that occurred at the military academy they visited the other day and of which they are also graduates. A pretty cool detective story which also further explores Reinhardt as a character, especially the relationship with his father.
"A Hundred Billion Stars"
One of the largest arcs of the Gaiden. It's about two military operations on the part of the Reich and the Alliance - the invasion of the Van Fleet system and the 6th attack on Iserlohn. Here we will also see more of the Rosenritters. This is the most action packed arc in the series, with a number of very memorable scenes and good characters.
"The Third Battle of Tiamat"
The Reich is planning a major invasion of the Alliance, due to recent successes in previous battles. This is where Reinhard will again distinguish himself, receive the rank of admiral and his flagship "Brunhilda". A very short but decent arc.
"Dishonour"
The only arc which takes place during the events of the original series. Siegfried Kircheis goes on vacation on the artificial resort satellite Kreusnach III before the Battle of Astarte, where he unexpectedly becomes involved in a drug trafficking case and is asked by the local police to help investigate the local criminal ordeal. A pretty original arc which I especially enjoy because it gives Kircheis more time to shine.
An absolute plus for me when watching this preque was more screen time for characters like Siegfried, Patrichev, Schenkopf and others. I enjoyed following young Reinhardt's story and his career growth. Same goes for Yang. This prequel does what every decent prequel should do - expand on the original material and create its' own story which is just as interesting to follow.
