The story is futuristic. It is about a boy who joins the military on a whim and finds out that it's the best place for him. The stories the narrator tells are of specific instances and adventures that he experienced. Between stories, time passes. Sometimes it's a long time, sometimes it isn't. If you've seen the movie, you know that there is an interstellar war with the bugs. The bugs are barely mentioned in the books until at least halfway through. So, the first portion of the book is about his training. The last part of the book is about the war.
The book itself is quite thought provoking. The author makes it a point to not only describe the military but to discuss it. I'll be honest, I was not expecting the philosophical aspect of this book. It raises many good points, and it's worth the read, but I was expecting more action in the book. I still really enjoyed the debate, and the story is none the worse for it.
One of the major points that Heinlein's fictional society makes is the right to serve a federal term (not exclusive to military). Once the term is complete, the individual is allowed to vote. That being said, he makes it a point to state that military personnel are not allowed to vote, veterans can. I think the point Heinlein tries to make is that people need to earn their rights, like the right to vote. This is where the US fails. As it stands, the only things you need to vote are 18 years of life and citizenship. Duty is the underlying theme and message of the book, as I see it. The idea that as duty dwindles, so does morality, is an interesting thought.
I'm not saying whether or not I agree with these ideas, but I don't know that it really matters whether you agree with them or not. They are there to make you think. So, if you're interested in thinking about a book and taking a look at society, then read this book. If you just want to read some stories that a man in a fictional and future military tells, then read this book. I believe it could be the beginning of some excellent discussion.