During chemistry class, Urkel is working hard as usual but Eddie would rather look at a nudie magazine. Disgusted, Steve exchanges plesantries with Mr. Sweeney and asks him for extra time during lunch time for his newest science fair project. When the bell rings everyone else leaves except for Urkel's rival, Dexter Thornhill who challenges him with hard questions. After a few minutes, Thornhill leaves dejected and more hateful towards him than ever. Minutes later, Steve goes in the storage room and the lab explodes. When he comes out, Mr. Looney asks if he did this and Urkel says no.
However, the student body doesn't believe Steve and blames him for blowing up one of the science labs at the school. As a result, he's suspended from school. The problem is that he was framed because he knows chemicals do explode, but no one believes him. The administration and the student body decides to put Urkel on trial and give him a chance to prove that he wasn't responsible for what happened, or he'll be expelled with his reputation tarnished. The only one who does believe his innocence is Laura and after consulting with Harriette at home. She agrees to defend him for the trial, not caring if she ruined the party the students will throw on the day Urkel is expelled.
With his academic future on the line, Steve is on trial at the school with Laura representing him as his defense attorney and his rival, Dexter as the prosecution. Mr.Shimata reads his charge as a judge and Urkel maintains his innocence. He sits down next to his friend and defense attorney. Before the trial can offically continue, a frantic Myra comes in the cafeteria and claims that she blew up the lab, not Steve. The student body, including Thornhill, Urkel and Laura are shocked by what she had just said. She even puts the blame on herself citing she's more than willing to be expelled from Vanderbuilt High to protect Urkel's standing at school. Steve takes her to one side and reminds her that she wasn't in school that day because she was waiting for the repairman to fix her television set. After a few shocking moments, Ukrel finally convinces Myra that she needs to let Laura do her job and clear his name.
Dexter's willing to do anything in his power to ensure Urkel is expelled from Vanderbuilt. He calls Mr. Sweeney to the stand and questions him. After he was done, Laura questions about him witnessing the lab explosion and Mr. Sweeney admitted that he really didn't know what happened. Next, Carl is called to the stand by Dexter and he admits to almost all the things that happened to him and his home, due to Urkel's clumsiness. However, he doesn't call it major because he mentions that Steve had saved his life before. Laura questions Carl about the whole thing as well. Her father admitted that Urkel always took responsibility for his actions. She askes him why he has denied the lab explosion and he said Steve was framed. Dexter tries calls out an objection claiming it was making him look bad. Mr. Shimata overrules it, citing a bad fashion sense on his part (having long hair and a terrible choice of suit) and the prosecution rests.
Laura quickly figures Dexter out, but she waits a while until she's ready because she wants him to hesitate. Instead, she calls Urkel to the stand to tell his side of the story. Steve tells her about him working on a new project for this year's science fair and treats his chemicals with respect. Waldo and Weasel sings to Respect until Shimata calls them to their seats. During the time, Mr. Looney comes in and he talks about mops, cotton vs sponge. The janitor seems interested and he goes inside the science lab storage to get a bigger bunsen burner to help his experiment out. The lab explodes while he was in there, but Dexter isn't convinced and tries to antagonize him. Mr.Looney is called next and he tells Laura about him leaving his sandwich on the bunsen burner to heat up for his lunch. Just before he had a chance to take it off and put it back in his lunchbox, he's paged by the administrators to clean up the roadkill off the parking lot. He leaves and the lab explodes. Laura asked him why he didn't come forward with the information. Looney admitted that he didn't want to get kicked out from an upcoming performance at the school's musical play, Carousel, because he's portraying a main character on it. Dexter had no questions for him and Looney tells him to cut his hair.
Near the end of the trial, Laura finally calls Prosecutor Thornhil to the stand. She questions his hate for Urkel and asked about the three years of winning 2nd place at the science fair. Dexter admitted that Steve(calling him by his nickname, Yuckel) always took 1st place, "a standing he claimed that he didn't deserve" and has hated him since then. Laura tells him that she knows he had the most to gain in Urkel's expulsion when he entered the science lab while her friend was in the storage room and Looney left for an emergency call. She mentions that Dexter has tampered with Steve's science project by adding exploding chemicals into his grafitti solvent and turning the bunsen burner up high. Then, he disappeared from the scene and he blew up the lab while framing Urkel for it. Thornhill denies her claims and challenges her to prove it. Laura proves it very well by using the black light to detect chemicals that can be on human skin for days, maybe weeks. She asks him to reveal his hands in the black light. Dexter reluctantly reveals his hands with the chemical stains on them and everyone is shocked. After the school lights are turned on, Dexter loses it and confesses that he was behind the lab explosion. He was sick of being in second place of last year's science fair to and deserved to win first place. He even mentions that he's tired of staying up until 4 a.m to outdo him, while Steve glances at a page and gets straight As. He condemns Urkel for being better than him, both in school and at the science fair. Mr. Shimata drops the chages against Steve(per Laura's recommendation) and the students throw a party for the defense. Urkel hugs Myra, Carl, Laura and Eddie for their support. However, after his confession, Dexter faces expulsion from Vanderbuilt High as Shimata escorts him to his office.
Later on in the end credits, Mr. Looney is cleaning out the cafeteria and started singing Frere Jacquere. Steve joins in on the singing.