The Wonder Twins agree to help Exxor defeat Darkon, a conqueror who has taken over Exxor and made its inhabitants his slaves. With the Wonder Twins captured by Darkon, Batman and Robin must free the twin heroes as the other Super Friends tackle Darkon and his troopers.

While sailing with Captain Pierre Marcel, the Wonder Twins and Gleek get sucked into a whirlpool. After receiving a distress call from the Wonder Twins, the rest of the Super Friends search for the twins, only to get pulled into the whirlpool themselves. They emerge in a Wonder Land that is below the Earth's crust. The Super Friends must now search for the Wonder Twins and rescue them.