- This article is about the episode. For the monster of the same name, see Marabounta (monster).
Jeremie, Aelita, Ulrich, and Odd are in Mrs Hertz's class learning about ants. She tells the class that these ants follow a sort of multi-agent system, and that one has a job to do and all work together upon doing it. It's under the impression that each ant "is rather clever" when in fact it is just a collective intelligence. Odd comments how even an army of Sissi's can be intelligent, and Sissi responds with "Very funny. Shut up, bird brain!"
Suzanne continues explaining that the ants working together accomplish many difficult tasks. They can even wage full-scale wars between different colonies. Ulrich asks about killer ants, but Suzanne dismisses the idea, stating that that can happen only in movies. Suzanne does, however, inform the class that army ants destroy everything in their path, and "you best stay out of their way." Sissi asks if the bugs live around them, but Suzanne says that they fortunately live in South America, where the phenomenon is called "Marabunta." Sissi comments how she's glad she lives in a country where there is no danger, to which Odd and Ulrich exchange a knowing look. As the class leaves, Jeremie looks on the ants, seeming to have an idea.
At lunch, Jeremie tells the group that he deciphered part of Franz Hopper's diary, coming across an interesting theory about multi-agent systems. It's very similar to the ways of an ant hill. But before he could go on, Odd stops him, telling him that Mrs. Hertz had already bored them with the ant stuff; "don't you start now" he says. As Ulrich, Yumi, and Odd laugh, Jeremie tells them that he's serious and that he might have found a way to weaken X.A.N.A.. He tells them to meet him at the factory to find out more, but everyone has something to do after lunch; Aelita has to finish her Italian homework, Odd has practice for a skateboard competition, and Yumi is going to the pool. Ulrich seems to not have plans after school, but upon seeing Yumi with William, he tells Jeremie he has to go to the swimming pool, too. Jeremie reluctantly states that he'll just deal with everything himself and leaves the cafeteria.
At the Factory, Jeremie starts with the program. At the swimming pool, Yumi watches as William takes an impressive dive from the high board and claps for him. As William swims over to Yumi, Ulrich walks to the diving board. Sissi watches him climb up, unsure of what he's about to do. William sees him and asks Yumi if she sees him about to dive, too. Yumi confusingly looks up at him. Once Ulrich has Yumi's attention, he begins to jump but slips on his feet and belly flops into the swimming pool. William comes and saves the day, while the rest of the class checks on him. One girl yells that he needs CPR. While it looked like Sissi was going to be the one to volunteer to save him, she sees Yumi going for it instead, but Jim comes to the rescue and pushes her away. Ulrich is shocked when he sees Jim instead of Yumi, who is smiling along with William. Ulrich coughs and Yumi asks if he's okay. Ulrich just gives a look and walks away.
Back at the Factory, Jeremie is about to complete the program and transfers the prototype, to which a black, globby sphere appears in the Forest Sector. Jeremie exclaims how Franz Hopper is a genius, and so is he.
At the dorm, Odd comes in to see a sulking Ulrich lying on his bed. Jeremie calls Odd and tells that he has something amazing to show them and that they need to come to the factory, and to not forget to pick up Aelita. Odd agrees and asks Ulrich if he's coming. Ulrich says he's not in the mood. Odd states that Jeremie will be mad if not everyone comes but Ulrich just wants to be by himself. Odd says he'll leave the both of them; Ulrich and his bad mood.
Odd and Aelita arrive at the Factory with Kiwi. Jeremie asks where Ulrich is and Odd says he's sulking. Aelita asks what his program is and Jeremie calls it the "Marabounta", his own creation. It is designed to be an effective weapon against X.A.N.A.'s monsters. Odd gives a short summary and says that while these little 'ants' get X.A.N.A. monsters, he'll leave them alone. But Aelita asks if there will be a real bug in the program, and Jeremie says that it's impossible. Odd suggests that they go and check it out for themselves and Jeremie agrees. He reassures Aelita that if anything goes wrong, he'll pull the plug, and Aelita reluctantly goes.
They are transferred to Lyoko and Jeremie tells them where to go, but they'll have to run because there are not enough machine resources to bring up the vehicles. He also tells them to watch out for the monsters, since "they never know" if X.A.N.A. will pop up. Upon seeing a Krab on his screen, Jeremie tells Odd and Aelita to follow it since it's heading in the right direction. The two see the crab rushing to the Marabounta and run after it.
At the pool, William asks if Sissi has been crazy for Ulrich for a long time, to which Sissi asks if he's been crazy for Yumi for a long time. William responds, "Forever". Sissi says he's only been at the school for a few months, but William asks if she believes in destiny with a capital "D", when two people have never met but are made for each other. Sissi says she doesn't believe in that kind of stuff and says she just wants to go out with Ulrich, as she's had a big crush on him since elementary school. But Sissi says it's "stuck-up Yumi's" fault, while William says it's "big shot Ulrich's" fault he can't date her. They both smile at each other, seeing as they both have a common hate. Yumi asks if they should go, since she's getting cold, leaving Sissi by herself.
Back on Lyoko, Odd and Aelita find the Marabounta swallowing up a Krab, and Odd calls it "pretty scary." X.A.N.A.'s monsters try to destroy the creature, but they all end up getting eaten as a result. Odd says that he "really feels sorry for X.A.N.A.." Aelita sends a visual to Jeremie, and Jeremie is satisfied with it. Odd says he invented a "nasty piece of work" but Jeremie still just looks on with satisfaction. Aelita says she's seen enough and Jeremie gives them the coordinates a tower that will bring her back.
However, as Aelita is about to turn, the Marabounta starts to circle around her. Odd sees she's in trouble and shoots at it, which did not fit well with the Marabounta. It starts chasing them and Odd just shoots at it even more, which makes it go faster. Jeremie wonders what's going on and Odd says it's coming for them. Jeremie says it's only supposed to chase monsters and Odd suggest he program eyeglasses for it. Jeremie asks if he did anything special, to which Odd responds that he just shot at it. Jeremie says that wasn't a very smart thing to do, but Aelita defends Odd, saying that the Marabounta was coming after her before he opened fire. Jeremie searches for a problem and finds out that because of the virus in Aelita, the Marabounta thinks she's one of X.A.N.A.'s monsters. As Jeremie is about to kill the program, the Marabounta becomes autonomous. It will submerge everything and everyone in its path. Jeremie tries to call for help, but Ulrich doesn't want to pick up his phone. Jeremie calls for Yumi instead and tells her what's going on, and she is just confused at the sound of this "Chattanooga". Jeremie just tells her to hurry to the factory. He says that Ulrich is being no help, but Yumi knows why and says she'll go and get him.
Odd and Aelita are lost, but they don't see the Marabounta anywhere either. Jeremie points them due west since the monster hasn't hit that part of the sector yet. In the meantime, Jeremie tries to find a way to destroy it.
Yumi finds Ulrich in his room, stating she knew he would be here. But Ulrich just wants everyone to leave him alone, especially Yumi. Yumi says that he shouldn't blame her for his own embarrassment and that no one asked him to spy on her at the pool. Ulrich, in disbelief, just says he "happened to be there." Yumi doesn't believe him and tells him to shelve his ego and that Odd and Aelita will be needing them. She leaves Ulrich and heads to the factory.
When Odd and Aelita sees that the Marabounta had already made it to the tower, Aelita backs away, afraid. Jeremie is still trying to find a way to stop it, and says that Yumi and Ulrich are on their way. Or at least, he hopes they are. Jeremie calls out to Franz Hopper to help, while Odd and Aelita are backing away from the Marabounta. Odd is shooting but every time he shoots, the Marabounta separates only to comes together again. Jeremie exclaims that Lyoko will be destroyed if this goes on any longer. The Marabounta manages to separate Odd from Aelita and she's cornered. But all of a sudden, a Krab materializes in front of Aelita. Instead of attacking like usual, it just extends its leg for her and Aelita holds on to it. The Krab throws her away from the Marabounta, sacrificing itself for her. Odd and Jeremie question its actions, whilst more monsters come and circle around Aelita and Odd. Jeremie realizes that X.A.N.A. is doing this because he can't let Aelita die. He needs her memory. Odd wonders why they are not attacking and Aelita comes to the conclusion that they are protecting her. Jeremie tells them to use them as allies to defeat the Marabounta, the one common enemy they all have right now.
As Yumi arrives, Jeremie wonders where Ulrich is and Yumi just says he's alone with his pride. She wonders what is going on Jeremie just tells her to go to the scanner room. As Odd fights along side X.A.N.A.'s monsters, Yumi is virtualized to Lyoko, and immediately goes after one of the Bloks. But Aelita stops her, stating that the Marabounta is the thing to go after. Yumi looks to it in disgust and wonders what it is. Odd says it was Jeremie's idea, but luckily, X.A.N.A.'s on their side today. Yumi says, "The world has turned upside down!" and joins in with the team, shooting her fan at the Marabounta.
Meanwhile, in Odd and Ulrich's dorm room, William comes to check on Ulrich, but Ulrich thinks he's just there to "humiliate the competition." William asks what is the problem and Ulrich points accusingly at him saying that he's his problem. William dismisses that conclusion and he tells Ulrich that him being around Yumi forces him to ask himself a tough question. William tells him that he should go and talk to Yumi "from the heart" or else William will do so first and then he really will be the problem.
Jeremie really has no idea how to defeat the Marabounta but Aelita comforts and tells him that he had no idea that her virus would be a problem. Upon hearing the word 'virus' Jeremie comes up with a solution that might work. As a lot of commotion happens at once, Jeremie is searching for his solution, the monsters and the heroes are still struggling to keep the Marabounta back, and it seems that Ulrich has finally come to his senses and is now rushing to the factory. Odd tells the lot of them they shouldn't let it take any head-way, or it will attack from the rear. Yumi says he's a good combat general, and Odd says it's all strategy. "The only thing stupider then a Blok, is two Bloks!" he says. Yumi rushes Jeremie with his solution but Jeremie is taking the time he has.
Jeremie tells them that he has inserted a deadly virus in one of Odd's arrows and that if he hits a sphere with it, the whole colony will be infected. Odd starts to aim, but Jeremie stops him. The virus will only work if he hits the very first sphere, at the heart of the Marabounta. Odd asks if he's supposed to get there on a camel's back, and a Krab behind him grunts, bowing it's head, offering Odd to get on top of it. The Krab and Odd charge their way with the help of Jeremie's guidance to the heart. With the other group, Aelita is almost engulfed by the Marabounta, but Yumi comes in to save her, only to get eaten herself and get devirtualized.
When the monster gets closer and closer to Aelita, she calls out to Jeremie. Ulrich then appears and jumps in front of her, fighting off the Marabounta with his katana. Meanwhile, the Krab is slowly making i's way whilst getting engulfed. Odd finds the heart and points right at it, making sure he's got a good shot. He shoots and destroys it in one go, exclaiming his victory. The sector is engulfed in light and the Marabounta disappears for good. X.A.N.A.'s monsters line up in front of Odd, Ulrich and Aelita. Ulrich holds on to his Sabor, ready for a fight, but the monsters instead bow to them, thanking them for the help. Aelita is amazed but Ulrich tells her to not buy it. Jeremie agrees, stating that those monsters are still their worst enemy, and that X.A.N.A. still wants her memory at all costs. The monsters leave and the warriors still look on in amazement.
Back in the forest near Kadic, Odd tells Jeremie to tell them about his ideas in advance. Jeremie says that he tried to tell them but they all had better things to do, even though Jeremie agrees that this was all just a really bad idea. Aelita only tells him he still found a solution, and that's what counts.
Sissi sees the group coming and makes a snarky comment, as usual, with Odd giving an insult right back. Sissi brushes him off and asks Ulrich if he's feeling better after what happened at the pool. Ulrich tells her to "cool it" and walks away, while she slyly smiles at him. Odd asks about what happened, to which Yumi and Ulrich just respond by saying, "Nothing!" Odd questions this answer, but smiles, as he might have a good idea of what happened.
- The original French name of this episode is the same as the English.
- This is the first episode in which X.A.N.A. sends his monsters to help out the gang in Lyoko; this and the next episode, Common Interest, are possibly the only 2 episodes in which the warriors and X.A.N.A. actually help each other.
- This episode marks the only appearance of the Marabounta. However, it is later mentioned again by Aelita and Odd in Down to Earth.
- Despite being previously mentioned in the episodes Log Book and Uncharted Territory, Marabounta marks the first appearance of the pool.
- This is one of the very rare episodes in which William and Sissi talk to each other.
- At one point during the pool scene/incident, William's hair is colored brown instead of dark blue.
- In one scene, virtualized Odd's purple forehead and cheek markings were missing. In another, the stripes on virtualized Aelita's cheeks were missing.