Flashpoint is the fourth and final book in the Unstoppable series. It was written by Gordon Korman, and published on August 26, 2014. In Flashpoint, the events of Unstoppable end. The final ingredient of the antidote is finally found, and Pierce is defeated for good. However, in the next series, Doublecross, a new villain arises, the Outcast, who challenges the Cahill leadership.