Mandarker is the third part of the 12th episode of season one in Dexter's Laboratory, which first aired on December 25, 1996. In this episode, Mandark, after having his lab destroyed by Dexter and Dee Dee, turns to magic to help him win the science fair.
Mandark is in his laboratory laughing when suddenly, Dexter breaks in being the smartest boy genius in the world and he makes a very scary and evil looking Dee Dee destroy Mandark and everything in his laboratory. Mandark wakes up only to realize that it was a dream. Mandark is sleeping in a bed that is amongst the remains of his blown up laboratory. Mandark starts crying over his destroyed laboratory and his recurring dream of it being destroyed. He starts talking to himslef about how horrible things have gone for him ever since, that fateful day. Then, Mandark has an idea. He digs through all of his old stuff and finds a book on how to do something. He starts thanking someone named "JoJo" for this book and starts laughing maniacally.
The next day at school, students and teachers are still loving Dexter and his genius. Dexter is just there basking in his popularity and acting cool and relaxed and happy with his life. Then, Mandark's voice is heard. Everyone turns to see him sitting in the deask way in the back of the room. Mandark tells Dexter that there is a school science fair coming up and that he is going to win. Mandark challenges Dexter to and and Dexter accepts. Dexter and Mandark get into a competition over who will win the science fair contest.
Later, Dexter is back in his laboratory sunbathing under a scientific sun light. Dee Dee runs up to him talking about the news of the science fair competeition between him and Mandark worried about him losing the competition. Dexter tells her not to worry about a thing. He says that he will just use another one of his old inventions. Dee Dee unsurely says that, that will be fine and leaves. Mandark is watching them on a screen from his laboratory of which he apperantly rebuilt again. Mandark says how there is no way Dexter will win, because he has something even more powerful than science and he laughs his evil laugh.
The next day, the science fair has begun and Dexter is mezmorizing the judges with his robotic arm. The judges are about to give Dexter the blue ribbon when Mandark comes up saying that he has something that he is going to use for his science fair. Something better than all of science in any way which is magic. Mandark uses the book which turns out to be a spell book and magically makes wizard clothing appear on himself. Mandark uses his magic to do amazing things which is better than science and then Mandark says he would like to show them who his little friend JoJo is and he uses his magic to create a giant portal to another dimension where a big green alien monster comes out of it and starts attacking everything. Mandark is laughing in evil of how he is winning, but then his amusment is quickly stopped when Jo Jo starts attacking Dee Dee. Mandark tries to get JoJo to stop, but he just keeps coming at her. Then JoJo picks Dee Dee up and eats her. Mandark and Dexter realize that if they don't save her soon enough, she will be digested and gone forever.
Dexter and Mandark team up with each other to stop JoJo. They combine the powers of magic and sience to defeat the evil monster and save Dee Dee. Their powers are so strong they destroy JoJo and Dee Dee comes flying out. Dexter takes out his robotic arm and Mandark uses his magic to make it reach up to Dee Dee where he grabs her and saves her Mandark gets rid of his spell book and all of his magic knowing that it's evil and should never be used. Dexter, Mandark, and Dee Dee all get along as friends. This is all until Mandark starts swooning over Dee Dee which freaks her out and makes her grab onto Dexter for protection making him yell at her, freaking her out making her mad at him and then the judges present the blue ribbon to both Dexter and Mandark making them argue over who deserves it more and getting the three of them to all get into a fight with each other making them enemies again.
Dexter is seen tanning under a lamp in his lab in this episode. In Season 2 in the episode Repairanoid, he is seen tanning under 7 lights while working. This could indicate Dexter does this occasionally, maybe to add shade to his complexion. Further evidence is seen in that episode when Dexter pops up from a toaster.
One of the elderly judges about to present Dexter with a ribbon is shown wearing a monocle and same exact shoes similar to The Mayor of Townsville from The Powerpuff Girls, both of whom are voiced by Tom Kenny.
Mandark said that it felt like an eternity since Dexter destroyed his laboratory, even though it was really Dee Dee who did that.
Although, Mandark may have said this since Dexter was the one who sent Dee Dee out to destroy it.