"Bokurano" is superficially a mecha story, but with an unusually strong focus on character development and a remarkably melancholic plot. It could be said to have lack of options as a major theme; for much or even most of the story the main characters find themselves pressed by circumstances into behaving in ways that they would rather not.
The protagonists, with a few notable exceptions, are middle schoolers that find themselves into a situation with implications that they will take a while to fully grasp.
It is a touching tale of loss, despair, and hopeless struggle against overwhelming odds.
The original story was published as a manga. The anime came later and has significant differences from the manga, even as it keeps the basic concept, most of the same characters and overall plot. Characterization changes slightly, and individual situations they find themselves in are in some cases greatly altered. The ending of the story itself is also changed.