Used Ink is the third part of the 5th aired and 4th produced episode in season 4 of Dexter's Laboratory, which first aired on May 16, 2003. In this episode, Dexter invents a pen that allows him to manipulate anyone into doing his deeds. However, Mandark soon gets ahold of the pen and uses it to his advantage.
Dexter is working on a very complicated mathematical equation and he is almost done with it until Mom calls him down to do his chores, so loudly and suddenly that he gets startled and messes everything up. Later, he is working on a giant robot until his Dad calls him down which startles him and causes him to mess up and the robot gets destroyed. Later, he is working on a science potion, but then Dee Dee calls him down and it startles him into making the entire thing blow up in his face.
Dexter is tired of this happening so he invents a pen where if he writes down the name of a person and what he wants them to do, it will control their mind and make them do it. So, he makes his mom, his dad, and Dee Dee do all his chores for him so that he will have the time to do all his work in his laboratory.
Dexter finished up everything he has done and has now taken a rest. Meanwhile, a floating spy camera comes down and looks at the pen and analyzes it. Then it flies back to Mandark's Laboratory and Mandark looks and sees what Dexter's got. Mandark says that he wants to get his hands on it. Mandark sneaks into Dexter's Laboratory unnoticed and steal the pen. Then, he writes down for Dexter to destroy his inventions and it controls his mind to do so. Then, Dexter sees Mandark there with it and he tries to get it back. The two of them fight over it until Dee Dee comes up and takes it away from them. Angry that her brother made her do his chores, Dee Dee writes down for them to do a bunch of girly things with them.
Dexter and Mandark are having a tea party with Dee Dee in Dexter's Lab which is now covered with pink, frilly, girly, stuff. Both boys begin to plot against her before Mandark steals it back from her and writes down for Dexter to do endless push ups. Dee Dee demands he stops and he agrees after writing for her to kiss him. Against her will, Dee Dee kisses Mandark on the lips and reacts in disgust. She then punches him in the gut and he drops the pen. After this, the argument breaks out into a huge fight and they all keep writing down stuff for them all to do and the fight just gets more and more out of control until they all pile up on top of each other and the pen rolls out of reach of all of them. Then, Monkey picks it up and writes down for them to give him bananas, fruits, coconuts, jungle trees, a tire swing, and a girl monkey. Monkey is laughing because he is now the one in control of them.