Dexter is trapped in his school by the janitor in revenge for making him work late everyday due to Dexter's habit of leaving the school last everyday.

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Past the vaulted doors lives the smartest boy the world has ever seen, Dexter, boy genius! He's constantly creating dazzling, worldsaving inventions in his secret laboratory. His big sister Dee Dee frequently wrecks his experiments. However, he has bigger problems such as his nemesis, Mandark, his brilliant rival at Huber Elementary School. Mom and Dad, of course, have no idea what their little angel is up to.