This book provides a fair foundation for teens' science fiction reading. After a horrible plague decimates the American (and probably the world's) population, martial law is instated by a self-proclaimed President, and the surviving population of the interior US is relocated to the coasts. There are renegades, however, who refuse to leave--and there is a settlement somewhere in the Midwest of people who want to live a different way. The story follows a group of teens who either leave the 'safety' of the military state to look for this settlement, or they just happen across the others who are looking for it.
The number of characters in the book is its only complexity, but for those who are unfamiliar with science fiction, that may be enough. (A simple plot is not necessarily a bad thing, and this is the first in a series, so perhaps it will get more complex.)
At least they stay in one time period--this is not the kind of science fiction that involves time travel, space travel, light sabers or moleculizers.
Teens who like this should try Stephen King's The Stand and Margaret Atwood's Oryx & Crake.