It's hard to write anything in-depth about the series without giving major spoilers, so almost everything will be here.
The giant robot (which is named 'Zearth' by one of the kids) is actually sent by 'the masterminds', who live in an Earth on a parallel dimension. Apparently they need more energy than they can get in their dimension, and what better way than to convert the matter of a dimension into energy? And what better way to decide what dimensions to 'eat up' than to have a competition between dimensions, to see which one is the stronger? That means that the strongest dimensions will feed off the weak ones and progress.
And their chosen way to do this is by sending a giant robot to an Earth, together with a caretaker. The last member of the crew of the previous Earth (that proved their worth by defeating all their 'enemies') comes with it. They need to contract a group of new pilots (preferrably teenagers, though I don't know why), and show them the ropes. It's really easy, actually: the robot moves with your thoughts, and all you have to do is to crush the 'central point' -- which is actually the pilot capsule of the enemy robot. Oh, and of course the robot is powered by your life force, so once the battle is over, the pilot drops dead.
It takes a few kids dying for this to sink in. And after every battle, after the pilot drops dead, the next pilot is chosen. He or she has until the next fight to reconcile themselves with their fate. And every kid is more of less fucked up. There's the elder brother who has to take care of his three younger siblings. There's the girl who was knocked up by her teacher (who posted pictures of their night together on the internet too -- classy act!). There's the sadistic megalomaniac. There's the nerd who is ignored by everyone.
Some go kicking and screaming. Some need a firm talking to before they decide to fight -- it is hard to motivate yourself if you don't care anything about the world. Some make their final arrangements with perfect clarity, and go out serenely. But once you pilot the robot, you die.
Then there are the adults. It takes some time before the government learns that Zearth is actually piloted by human children who fight for the preservation of the world. This is kept hidden from the general public. A research project is started to see how the children can be freed from their contracts -- but actually the researchers (the head one being the mother of one of the future pilots!) are more interested in uncovering the secrets of Zearth and using that in their own inventions... When they do, though, they are in for a surprise: one of the secrets of Zearth is some sort of wireless power transmitter, and when it is built and turned on, the masterminds gladly start to drink the Earth dry through this transmitter!
There are some adults who care for the children and who support them, but they all meet with a more or less grisly end. So in the end, it's all up to the children themselves to motivate them to keep fighting and to keep everyone in this dimension alive. It takes some cunning and a revolt against the sadistic Dung Beetle to gain control of Zearth, and to end the fights for this Earth once and for all.
This is a hard series to watch. Everyone and everything is fucked up, and there are some hard choices to be made by the teenagers -- and they have to make those choices alone. There is no-one to help them. The sense of isolation and desperation is expressed quite well throughout the series. That sometimes makes for unpleasant scenes.
Technically, the series is not very special, but it serves its purpose very well. The 3D CGI is, of course, quite nicely done, but the character animation is not that fluid or special. But that's not where the focus of the series lies anyway -- it's not an action-packed show (apart from the robot fights). Voice acting is sometimes a bit uneven, but there are a lot of characters anyway. All the primary characters sound quite good.
- Honest and brutal, very good at summoning a certain mood;
- Good ending.
- Brutal, difficult to watch at times.
All in all, I'll give it an 8. It's not an easy show to watch, but the storytelling is top-notch. Endure it, the rewards are there.