At the end of the last book, Plum Island was attacked and the Variants captured Dr. Lovato thus putting humanity's last hope in jeopardy. Kate was working on a new Variant killing weapon, Kryptonite, and Plum Island was the key to that weapon being ready on time. Of course Master Sergeant Reed Beckham is not going to stand around while the woman he loves is taken to her death by a bunch of mutants so this book starts off right in the action with a rescue mission that is a race against time.
Extinction End - all it takes is all you got.