Dexter’s parents discover that he is making false excuses to get out of physical education and now Dexter must pass a physical exam if he wants to pass P.E. and move up a few grades.
Monkey’s birthday party is interrupted by Barbequor (Robert Ridgely), an intergalactic villain who plans to eat the planets, and his sidekick, the Silver Spooner (Rob Paulsen).
Suspicious that Dee Dee has not entered his lab even once on a particular day, Dexter shrinks himself and enters her room to spy on her. He soon becomes an unwilling character in Dee Dee’s doll story, but he believes the story to be real.
When his mother falls ill, Dexter creates a robot to do her chores. Dee Dee gets hold of its remote control and wreaks havoc, forcing Dexter to create a second robot to counter Dee Dee’s.
Dee Dee and her friends, Lee Lee and Mee Mee, enter Dexter’s lab and wreak havoc. Dexter clones himself to stop them, but the girls too make clones of themselves.
Dexter travels to the moon and finds that aliens are planning an invasion on Earth, and he uses his Robo-Dexo 2000 to stop them. It turns out that they only were interested in purchasing sweaters for the winter.
Since his studying is interrupting his favorite TV shows, Dexter creates a helmet to absorb the shows directly. However, the plan backfires when he randomly snaps into television-based outrages.
After Dee Dee declares that she can beat Dexter in any game, Dexter challenges her to a series of games in his lab, though he prefers to win by cheating.
After watching an Action Hank movie on television, Dexter grows a synthetic beard to try to be cool, but the police mistake him for Action Hank himself.
Dexter makes his own Major Glory action figures so that he can join the neighborhood’s Major Glory gang, but gets into serious trouble when the rest of the gang finds out that none of his action figures are original.
Dee Dee dares Dexter into building her a giant robot similar to his Robo-Dexo 2000. They then team up to try to save an island’s population from being destroyed by an erupting volcano using their giant robots.
