It's a sunny morning, and Dexter gets up cheerfully, shouting that the first day of school has finally arrived! He takes a bath, brushes his teeth, and chooses a brand new tux to wear to school. In school, as Dexter walks to his class, his teachers greet him and mention that it's good to have him back. In the classroom, Dexter sits down as his teacher welcomes the students. Suddenly, a shadow like figure raises his hand behind Dexter and tells the teacher that the answer is Hydrogen. Mr. Levinski, the teacher, responds that it is the right answer to the question he was about to ask. Dexter is shocked as Mr. Levinski then introduces the new student and tells him to come in front of the class. The person who answered the question comes in front and Mr. Levinski introduces him. The boy corrects Mr. Levinski to simply call him Mandark! As soon as he says his name, lightning strikes behind him. Mandark asks if he can relocate himself to the front of the class and Mr. Levinski agrees. Mandark sits next to Dexter in front of the class and after a moment, looks at him and tells him that he can read Dexter's thoughts and is indeed smarter than him. Dexter is flabbergasted. Just then, from the speakers, the Principal makes an announcement to welcome the students, and suddenly yells for the test studies to begin. Mr. Levinski starts to test the class by asking them to write an essay on what they did during the summer, and later solve equations. Dexter gets only an 'A' in all of them, whereas Mandark gets an 'A+'. When Mandark answers all the general knowledge questions before Dexter in class, Mr. Levinski sheds a tear at how incredible Mandark is. Dexter runs out of the class, screaming. He opens his locker and looks at a photo of Einstein he has stuck inside. He weeps and apologizes to the photo for failing to follow on his mentor's footsteps. Just then, Mandark comes to Dexter and asks him about his laboratory. Dexter thinks that the laboratory is the only thing that Mandark doesn't have, and by showing it to Mandark, Dexter will still be the smartest guy.
He takes Mandark to his laboratory shows him around. Mandark looks around and yawns. He teleports Dexter from the laboratory. They arrive in a huge scientific facility, and Mandark welcomes Dexter to his laboratory. Dexter is amazed at Mandark's laboratory. Mandark tells him that his laboratory has experiments covering every aspect of science known to man, and therefore it would be a waste to use energy on Dexter's laboratory, instead of Mandark's. Mandark gives Dexter an ultimatum to either work for him or terminate Dexter's laboratory, and teleports Dexter back to his laboratory.
That night, while it is raining outside, in his laboratory, Dexter reaches for the main power switch. As the computer questions Dexter on what he is doing, Dexter bids goodbye to his love and pulls down the lever and sadly shuts down the power. Meanwhile in Mandark's laboratory, Mandark's robot sees the energy readings drop from Dexter's laboratory and rise in Mandark's. Mandark cackles evilly, while Dexter weeps in his bed.
The next morning, as the alarm rings, Dexter gets up gloomily, and gets ready for school. When Dexter arrives in school, he greets his teachers. His teachers give him the cold shoulder instead and ignore him. Just then, Mr. Levinski yells to the faculty on Mandark's arrival. Everyone run towards Mandark and start greeting him. Dexter starts to cry, when Dee Dee comes up to him and yells 'Hi Dexter!' Mandark hears Dee Dee's voice and pushes away from the crowd to see her. When Mandark sees Dee Dee, he immediately falls in love with her. After Dee Dee leaves, Mandark runs up to Dexter and asks him who she is. Dexter is annoyed and tells him that it's just his sister Dee Dee. Mandark begs Dexter to set him up with Dee Dee on a date. Dexter first thinks that after what Mandark did to him, he shouldn't do him any favors. However, he gets an idea on his head when he realizes what Dee Dee would do to Mandark's laboratory, and tells Mandark that he will set him up. He starts whispering in Mandark's ears, telling him what to do.
Later, back at home, Dexter knocks on Dee Dee's door and runs. Dee Dee opens the door and sees a trail of candy on the corridor. She starts following the trail by eating the candy along the way, and soon lands in front of Mandark's house. Mandark greets her and tells her that there is more candy upstairs if she wants. Dee Dee agrees and they go to Mandark's laboratory. Mandark hands her a plate of candy and Dee Dee starts to eat them all. As she is eating, Mandark puts a CD in the player and asks her to dance. Dee Dee gets excited and grabs Mandark to dance with her. She starts wrecking Mandark's laboratory with her dance moves. Mandark orders his robots to stop Dee Dee, but Dee Dee uses her dance moves and knocks them off without even knowing it. Meanwhile, Dexter relaxes in his laboratory, as he sees from his computer screen, the explosions coming from Mandark's laboratory. He takes a glass, and looking at his computer, gives a toast to Dee Dee for being a good sister. His computer tells him that it's good to be back.