A comet is spotted heading towards Earth, and we follow a large cast of assorted characters as they prepare for a hit or near miss. Of course, most people expect the comet to miss the Earth, but it's obvious from the start to the reader that it won't.
The book is long but I didn't find it a chore to finish, and enjoyed all of it. The story includes a fairly lengthy build-up to the impact, the event itself and the short-term and mid-term aftermath. There are a LOT of characters, with some being described in more detail than others depending on their importance to the plot, but there is a handy list at the beginning of the book to help the reader keep track of who is who. This variety of characters allows us to see the event from lots of different perspectives, which I found interesting. The story is centred in and around the LA basin area, but scenes involving the crew of a space station and various officials does give some idea of conditions elsewhere in the US and abroad (but without much detail).
This is a good example of apocalyptic fiction. It covers all phases of the disaster, is well written and gives a feeling for what things could be like if a world-wide catstrophe were ever to happen.