Zak and the other young heroes must descend deep underground to face the creature that wears the lava crown and commands the troll army--can they rescue their mentor, Dr. Hoo and the captive humans, or has the doctor betrayed them?

On the morning after the troll attack, Pablo and his friends wake to find that a dark tower is rising from the ground, and that only they can stop the army of evil that is about to invade Zion Falls.

BATMAN has been kidnapped by the RIDDLER! The crook has left behind one of his infamous riddles, addressed to ROBIN. If ROBIN can solve the puzzle, he will locate the CAPED CRUSADER. But that first riddle leads to another. Then the RIDDLER turns off all the lights in GOTHAM CITY. What, ROBIN wonders, is the point? When will the riddles end? If the BOY WONDER cannot find his partner by the next sunrise, BATMAN will be gone forever.

Meteors fall above the town of Zion Falls, and below the town an ancient evil awakens--caught in the middle are four friends, who must use their new powers to defeat the troll monsters and save the world.