Just to for warn any one who hasn't read the first two books, there's going to be major spoilers from both of those books.
In book one we saw Alex, an average 14 year old teenager from Cedar Falls Ohio, after having an argument with his mother he's left home along while his parents take a trip to Illinois to see his uncle. While he's home along a super volcano erupts and puts the world into turmoil. After the initial shock he decides to try and make the trek to Illinois to find his parents. On the way he runs into trouble and even though he escapes he is still injured in the process. Fortunately he comes across a girl and her mother living on a farm, surviving on corn that they dig up from under the ashfall. The girl, Darla, and her mother nurse him back to health. Then a few weeks later, while Darla and Alex are in town trading for supplies, their farm is attacked. Darla's mother is killed and the farm is burned.
With nothing to keep Darla she goes with Alex to Illinois in search of his parents. They have more adventures, some of which involve a FEMA refugee camp that's run like a prison camp, and all the while a young love blossoms between Alex and Darla. Once they finally reach his uncles house, half dead from their journey they find out that his parent's have left to go look for him.
The second book takes place six months later, in this one we watch as Alex and Darla to strike out in search of his parents. While hunting for his parents they run into a gang of cannibals, and Darla ends up captured. Now with Darla captured he must first try and rescue her and then search for his parents. While he's trying to find out where they took Darla he again gets stuck in a FEMA camp, and amazingly he finds his parents stuck in the same camp.
After they escape from the FEMA camp Alex is dead set on rescuing Darla even though his mother is against the idea. During the rescue, Darla and Alex's father are shot, Darla survives but unfortunately his father does not. To make matters worse, when they return home they find that the neighboring town attacked, and took over their town. Now the town citizens are living in a tent city around Alex's uncle's farm.
As with the first two books, this one is told in a first person format from Alex's POV. The book opens up with the towns people of Warren getting ready to mount a counter attack to retake their town. The battle doesn't go down the way they planned and a lot of people end up dead. That night Alex gets a few volunteers to go to Stockton and lead a sneak attack and take over Stockton while most of Stockton's people are raiding Warren's food reserves. Alex takes the leader of Stockton captive and ends up make a deal to get Warren back, although he has made an enemy out of Stockton's leader, and he's bad dude.
Overall this one is as good as the first two, it really shows the growth of Alex and Darla's characters. Unlike the last couple of books this book told over the course of two years. Even though at first Alex doesn't like it, people tend to look to him for leadership even though he's only 16. I enjoyed watching him grow into that role, and accept that people would follow his lead.
Through all three book Alex and Darla's romance has been a side story. In the first book we watched the start of their romance, in the second book Darla gets kidnapped and Alex runs into the beautiful Alyssa, and even though Alex doesn't have feelings for her, that doesn't mean Darla has to like her. In the third book we see their romance take the next step as they build a safe community together.
One of the big controversies is between Alex and his mother, see his mother is blaming Darla for the death of Alex's father. His father went with Alex to rescue Darla in the second book and ended up getting killed, and now his mother is having a hard time dealing with that. Alex is now caught between the two women he cares most about, all he wants is for them to get along.
I didn't really feel like there was one main mission in this book, like in the first two books. That didn't detract from the book however, felt it was very well done, and I loved watching Alex overcome one obstacle after another. Just as in the first two book this one had a plenty of brutal scenes, showing mankind at it's worst. This book is classified as a "young adult" but if I had kids I don't think I'd let them read this until they were more mature. That being said, I think Mullin does a good job showing what the world would be like if a super volcano really did erupt.
Oh another thing, the ending was awesome, not going to tell you anything about it, just know it ends well.