It's a classic "only in anime" premise. A group of kids on a summer learning vacation think they're all signing up for an advanced video game but it quickly becomes clear that the giant robot they were riding in to fight alien invaders is very much real. What's worse, they learn that each of them will be required to pilot in turn - and only once, because once they defeat the enemy, the pilot dies. It's a classic giant robot battle, but the young kids all know that they will die once they get behind the controls.
We still haven't found out what exactly this invading threat to the world actually is - monsters show up and the kids are summoned. The pilot is decided by whoever personal chair gets picked and they are thrust into battle, though they usually have some time when they know that they've been picked but before the actual fight begins. That's why we get a look at their life and how they spend their last few days. What they say good bye to and (ultimately) why they choose to fight instead of running away.
Obviously, the show has a rather dark tone to it. Not unlike the creator's other manga-turned-anime, Shadow Star Narutaru, the show gets rather surprisingly dark and brutal, putting it's children through the ringer. The difference here is that the show is expected to burn through the cast at a steady rate and punches don't need to be pulled since it can just move on to the next kid. Without anyone to stick around and have to take all the abuse, the tragedy doesn't really pile up to unbelievable levels either - aside from the basic "giant robot pilot sentenced to death" conceit. Perhaps it's because I haven't ready the original manga, like I did with Narutaru, but Bokurano doesn't seem to feel as stifled or constricted in fully exploring what's going on with its characters.
It's unclear whether this will all build to something cohesive and it is admittedly rather formulaic so far: a new pilot is picked, they muse about their life and we get a glimpse of everything that was going wrong around them (and sometimes what was going right) and they ultimately decide to fight to protect something they love - or they screw up horribly and we find out that they should have been protecting someone. There's also the looming specter that the director of the anime came out and said he hated the original manga and ended up getting canned from the project for it. Not exactly inspiring a whole lot of confidence, but it's still an enjoyable show with a lot of potential. Any show willing to kill children gets some points.