As the book begins, a meteor is about to hit the moon. Once it actually does so, and knocks it just enough out of its regular orbit, things start happening: first major things such as tsunamis, and then less noticeable things, such as a change in seasons. Eventually, everyone is on their own with no resources from the city, state, or government.
I thought this book was really interesting and actually realistic about what might happen to people when they had to fend for themselves. I highly recommend this book and the next two to anyone who likes survival stories. The second book, The Dead and the Gone, is very good, too. It takes place in the same time line as Life As We Knew It (starting from the meteor), and follows Alex, who must protect his sisters in the very urban New York. The third book, This World We Live In, has Alex and Miranda meet.