The comic begins with an introduction on Melvin being a tattletale. Every day he continuously tattles on people for breaking rules, which were none of his business which causes apparent pain and misery to the rule breakers since they don't seem to mean any harm whatsoever by it.
One day he tattles to the cops that a robber robbed the bank after seeing the robber with a bag of money breaking out through a window. The cops thank him and arrest the robber. The press is impressed with what he did and soon he becomes a hero. He becomes so popular that he decides to run for mayor, eventually winning in a landslide victory. His secretary congratulates him and says that he is the youngest mayor in the city. Melvin agrees and says there will be some BIG changes around now.
Soon enough, Mayor Melvin has created a bunch of new laws that are apparently dumb (almost forbidding people from doing almost anything). People (and animals) are soon getting arrested left and right for breaking these rules no matter how minor they are (one guy is even arrested despite that he didn't do anything, but he still got arrested because he said that he didn't do anything). They soon are all sent to the jails (Each one specific by gender, age, or animal. Even Super Diaper Baby and Diaper Dog are sent there for some odd reason).
Soon Melvin is then told by his secretary that all the jails are completely full. Melvin decides to build a robo-jail and catch the lawbreakers himself. He creates the Tattle-Tron 2000 to prove it. He soon begins catching lawbreakers even people that weren't even intentionally breaking the law. He then soon heads for the school. The principal is told by some students that Melvin's robot ran across the soccer field (the grass there was apparently just planted recently) and squished the gym teacher. They all call out for Captain Underpants. When Captain Underpants comes and asks what's up, he is caught by Melvin who places him under arrest for busting the roof (this is how the Captain came in).
The Captain escapes and although he wants to fight the robot, he does not want to hurt the incarcerated people inside it. He soon gets an idea while passing by a prune juice factory. He then takes a huge bottle of prune juice and pours it into the mouth of the robot. This causes the robot to poop out all of the prisoners (through a muffler at the rear end of the robot). Eventually Melvin becomes the only person in the robot. The Captain then punches Melvin out of the robot (He was in a glass containment head at the top of the body. He also yells that he is telling on the Captain for that.). The Captain then puts him in the "Jail for Dumb Kids" and flies off to complete one more thing. Everyone congratulates the Captain for saving the day (but not right after Melvin says "You guys are so immature!").
The last pages then shows Captain Underpants freeing all of the citizens and animals that Melvin sent to the jails during his reign. The people and animals cheer for the Captain for freeing them and are happy that they are now free (the Captain also says "Tra-La- Laaaa!" shortly after) as the comic ends.