The episode starts off with a couple of rats trying to get some food. The other rats don't let one of the rats help, so he leaves, and finds some apples. However, one of X.A.N.A.'s spectres appears and takes control of the rat. The rat returns and attacks one of them.
In the library, Jeremie is explaining to Aelita why they must stay silent in the library, Aelita says "Oh, I guess it's because you need silence to focus on what you are studying" and Jeremie says "Yeah and not miss the match point." Ulrich and Odd are playing with a paper ball, to see who gets the match point, Odd stands up and hits it, Ulrich leans back, and holds on to the table with his leg and taps it and gets it to the other side, and he wins. But Ulrich loses balance and falls down. Jim comes and yells at him and tells him: "Ulrich! Is that what you call studying?!" Ulrich laughs, while Yumi is trying not to show Jim that she is laughing. The principal comes in and Jim grabs his ear, he doesn't notice it is the principal and lets go. The gang starts laughing. The principal came to look for Jeremie, and he needs to talk to him.
In the sewers, the rat that has been possessed by X.A.N.A. bites the other rats, spreading an infection which helps the malevolent virus gain an army of rats under its control. Outside, Jeremie comes out and tells Ulrich, Yumi and Odd everything about what the principal told him. He says that the school psychology has been looking at his progress and that he is going to see that he gets moved to another school if he has a really high IQ. Odd says "Hold on! You mean he is going to send you to a school for future Einsteins?" Jeremie says "Yeah…", and that his folks aren't going to pass a great opportunity like that.
In the sewers, X.A.N.A. has taken over a big army of rats; with the main rat commanding the rest of the pack.
Outside, Jeremie and the gang are talking about how to flunk the test, so he doesn't get transferred into another school. Odd says "What’s the big problem? You flunk the test and they will think you are a total moron, no one will ever know!" Jeremie tells them that those kinds of test are full of trick question, and you need to be clever to flunk that test, Yumi says that he is the cleverest kid around so there is no reason to worry, and Jeremie tells them that he will try his best. Yumi leaves and tells Ulrich to keep her posted.
In the library, Jeremie is taking the test, as Ulrich and Odd go and eat. Meanwhile three rats cut the electrical wires and everything is dark. Sissi in her room says: "What kind of a two-bit school is this?" There is a pack of rats in her room, she manages to get away. In the cafeteria everyone is clowning around, Ulrich tells everyone to quiet down, then the rats come down from the ceiling and start to attack. The kids try to leave, but they are surrounded by rats outside. Ulrich calls Yumi and tells her to come to the school. Ulrich then tries to call Jeremie, but the psychologist doesn't let him answer.
At the cafeteria, the kids have made it outside using the extinguishers to scare the rats away. Sissi is on the other side of the rat army, but lucky for her, Yumi is coming on her bike, she gets Sissi, and goes to where Ulrich and Odd are. Jeremie calls Yumi and tells her that they will meet in the factory, Yumi tells everyone to go to the science building, everyone goes up to the roof. Yumi says "We have to get to the factory, Jeremie said he will meet us there!" Ulrich says "Great idea, but how do we get there?" Odd finds a hose, and they go down to the ground from the ceiling. They must go over the wall since the sewers are packed with rats.
At the Factory, Yumi virtualizes Ulrich and Odd, as Jeremie is going to the factory. On Lyoko, Ulrich and Odd don't see Aelita; Yumi realizes that she typed in the wrong coordinates, Odd says that it's okay, and that they will run. As they run, they see Aelita being chased by two Krabs. As they close in, everything starts tilting. Yumi thinks it’s her mistake but Aelita says that X.A.N.A. has changed the slope of the plateau. Odd, Ulrich and Aelita let go, but Aelita makes a place for them to land on using her Creativity before they can fall into the Digital Sea. The Krabs charge in right after them. The two Krabs shoot at Aelita, but Ulrich stands in front of her and takes the hits. Odd tells Ulrich to protect Aelita, so he could attract their attention. Odd destroys one Krab but the other shoots a rock he was holding on to, and he falls toward the Digital Sea, but the Krab devirtualizes him just in time.
Jeremie arrives at the factory, and tells Yumi to go to the scanners. At Kadic, the students are on the ceiling and the rats have managed to get to them, and try to chew through the door. On Lyoko, Ulrich is protecting Aelita, but he gets shot twice. He tries to destroy the Krab, but it devirtualizes him. Just as the Krab is about to attack Aelita, Yumi destroys it. At Kadic, the rats have managed to open the door to the roof. Yumi takes Aelita to the tower. In the library, the psychologist says "Say… say cheese!", as a group of rats enter the room. Aelita enters the tower and just as the rats are about to attack the kids, the tower is deactivated. Jeremie then launches a return to the past.
After the return trip, the gang are in the library talking about the test that Jeremie is going to take. Yumi says: "Hey Jeremie, have you figured out a way to flunk the test?" Jeremie replies by saying: "Well now that I know all the questions, I shouldn't have any problems giving all the wrong answers." Ulrich says: "Are you sure it’s going to work?" and Jeremie says: "I’ve got a foolproof way of flunking. I'm going get all my answers from Odd!" Ulrich and Yumi start laughing, and Odd says: "Yeah, yeah, laugh your heads off all you want, but who knows you might be in for a big surprise!" The episode ends on that.
- The original French title of this episode is Enragés.
- This was the first episode to feature both Jean-Rémi François and Bruno Merle as writers.
- The title of this episode could be a reference to the Black Death, an infamous epidemic virus that killed millions of people. Ironically, it was caused by rats, similar to these in this episode.
- Odd makes a reference to the Rat Pack when he says "Welcome to the rat pack", a group from the mid 1960's made up of Dean Martin, Frank Sinatra, Sammy Davis Jr, Joey Bishop, and Peter Lawford.
- The gifted school mentioned in this episode would later be mentioned again in "Franz Hopper".
- This episode shows that X.A.N.A. can possess living beings.
- X.A.N.A. can possess rats in this episode, yet he doesn't seem to know that rats are afraid of lights.
- This episode was adapted into book format in "The Adventure Begins".
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