An alien technology called Aldnoah was found on Mars, which is where the mecha (called Kataphrakts) part comes in. Mars has Kataphrakts that are powered by Aldnoah Drives, and Earth develops their own Kataphrakts in response. The Earth versions are generally much weaker due to the lack of Aldnoah power, and the Mars versions have all kinds of special abilities. Not just anyone can activate and pilot Aldnoah-based craft, which has stratified the Mars society into those who can (blessed by the royal family) and everyone else.
There is a whole host of characters on both sides. Princess Asseylum of Mars attempts a peace mission to Earth, but is betrayed by way of an assassination attempt, which is used as an excuse to invade Earth. Slaine is a Terran who fights with the Mars forces, conflicted in his loyalties. Inaho is a student nearing graduation on Earth, forced into service when the Mars Knights attack. Plus an extensive supporting cast on both sides.
I really enjoyed the first twelve episodes. Learning all the backstory and meeting the characters happens in the context of Inaho and friends meeting the Princess and trying to find a way for her to stop the fighting. That doesn't go well, largely due to the same Mars Knight faction who betrayed her in the first place. There's a lot of character conflict and growth: the Princess has her illusions about her own people punctured, various characters deal with losses inflicted in the fighting, the ex-students have to grow up quickly, and so on. I particularly liked one of the sub-plots about an Earth Lieutenant's struggles to overcome the PTSD he suffered in the original invasion back in the 1990s. There are issues...it's hard to believe just how good Inaho is at fighting superior forces, or how Slaine manages to survive and even prosper on the Mars side despite his lowly Terran-born position...but suspending disbelief wasn't that hard.
But then there's the final twelve episodes. (Spoiler warning from here on out.) At the end of episode twelve, it appears that both Inaho and the Princess have died, in large part due to the actions of Slaine. I was looking forward to seeing how Slaine, and all of the supporting cast, would deal with the fallout from those deaths, and how it would affect the larger story of the war. But instead, by some miracle, both of them survive (though the Princess is comatose). There are changes: there's a stand-in for the Princess so no one knows she's hurt, and Inaho gets a cyborg eye. I can't say that this surprised me...heroes nearly dying then coming back is a pretty common thing...but it's disappointing to see the same old ground covered here.
What a wasted opportunity for a show to explore something other than the usual "two really strong fighters go at each other against the backdrop of war" plot. Had they actually killed off Inaho and the Princess, and used the second half of the series to explore the result, it would have continued to cover interesting story ground and character evolutions. But the deus ex machina survival for Inaho and the Princess, and the following Inaho-vs-Slaine maneuvering and battles, shifts Aldnoah.Zero from "interesting new take on the genre" territory to "just another mecha anime."
Before long, Slaine miraculously rises to high office in the Mars Knights, and Inaho is back to winning battles against greatly superior Mars forces by sheer force of his intellect (and cyborg eye). Outside of Slaine spiraling out of control, the characters are pretty much static...no one else really changes much in those last twelve episodes. It's mildly interesting to see how they finally wrap up the war, but I can't imagine the end result surprised anyone. All the things that were hard to believe in the first half of the show are put on full display in the second half, while the parts that I liked seem pushed to the background.