Thunder and Sarah Royce have a new school, new friends and whole new set of problems. Join the Royce twins and their father, Air Force Major Harold Royce, on another harrowing adventure, one that introduces them to new worlds and new dangers. An old enemy raises its head again. The birdmen of Saludia are intent on revenge, and their nemesis planet, Celestia is in their sights. Nothing is going to stop them from destroying it now that they've come into possession of armed, nuclear warheads, courtesy of the United States Air Force. It's up to Thunder, Sarah and their new friend, Emily Spencer from the Alter World of Atlantis to work together to stop the Saludians' evil plans.