It's about John Perry, a 75-year-old man who's recently buried his beloved wife. He would ordinarily be looking at a normal old age on Earth—this still involves the prospect of depressing diseases, breakdowns, and death, sudden or slowww—but in his time there's another option: you can sign up for the Colonial Defense Forces, go offworld and be (somehow) rejuvenated for the (very) dirty work of defending human space colonies against the hosts of bugeyed monsters that desire those worlds for themselves. (A distressing number of the aliens have developed a taste for humans, even featuring recipes on the Alien Food Network. Yeesh! Alien Rachael Ray! Yummo!)
Many people have remarked on the similarity of Scalzi's work to that of Robert A. Heinlein, and that's certainly true. Scalzi even makes a note of his obvious debts to Heinlein in the acknowledgments at the end. Like RAH, while the novel is set in the far-flung future, there are still recognizable features of the present. (When all the geezers gather together for a meal during their flight out from Earth, one character remarks "It's like Wednesday morning at the world's biggest Denny's.")
Quibble: It's difficult to buy the underlying human-vs-alien conflict in the book. Given the realities of the timing of stellar and biological evolution, how likely is it that multiple species will find themselves simultaneously in competition for the same planets in the same corner of the galaxy? Think about the likelihood of your entire kindergarten class showing up by coincidence 50 years later in line for It's a Small World at Disneyland. The book's scenario is way more far-fetched than that.
So you need to do that whole suspension-of-disbelief thing, OK? No problem here. Maybe it's explained in the sequel. Or explained away. I'm cool with that too.
Is it unforgivably old-fashioned to like an author mostly because he writes like an author you revered in your younger days? Guilty as charged.