Humanity is expanding beyond Earth and colonizing the stars. Only they're not alone, and a lot of other species have similar habitat requirements, so fighting over planets is always necessary. The Colonial Defense Force recruits old people from Earth - people who'd expect to be dying soon anyway - and promises them a new chance at youth (by mechanisms not revealed to them) in exchange for military service defending the colonies. So John Perry walks into a recruiting office on his 75th birthday and begins a new life as a soldier in space...
This series is everything I could have hoped it would be. The military-fiction aspects of it are engaging without bogging down in technical details, and anything not important to the plot or character development is left out. The science is plausible, with a minimal amount of handwavium required. The aliens are truly alien rather than feeling like humans in makeup and prosthetics. And the dialogue never fails to be snappy and fun to read - the characters are witty and the overall sense of humor is sharp.
I'm going to have to read everything else Scalzi's written, I think. (At least novel-wise - I've been keeping up with his blog, but ten years of archives are a bit much particularly when so much of what he writes there pertains to current-at-the-time events.)
Next up is the rest of the Dresden series (up to current, anyway). Sora is finishing them off right now, and I need to catch up so we can talk about them.
