The world is a monster madhouse, and Scooby and the gang have been fighting for their lives, for every sunrise, for every moment. But they've had enough-it's time to stop reacting and start solving the mystery of who turned all of humanity into mindless hordes of bloodthirsty creatures. A single clue leads the gang and their Mystery Machine to Seattle, on the trail of one of the four scientists who infected the world with nanites. Where to start? Velma has a plan, and watch out, because whenever Velma has a plan, things get very dangerous very fast.