In Mrs. Hertz's class, the students are studying ant behavior. Ulrich asks if killer ants exist. Mrs. Hertz tells him that in South America, army ants will go on a rampage, killing everything in their path. The event is known as "marabounta". The bell rings and everyone leaves, except for Jeremie, who is fascinated by the ants. At the cafeteria, Jeremie tells the others that he has decoded part of Franz Hopper's diary and discovered a program which he hopes will weaken XANA. He says it follows the properties of an ant hill, but at the mere mention of the word, Odd backs out, already bored to tears with Mrs. Hertz's lecture. Aelita says she can't go because of Italian homework. Yumi tells him that she has swimming. Jeremie asks Ulrich, but he responds, "swimming pool", eyeing William. With no one to help him, Jeremie heads over to the Factory to deal with it himself.
At the swimming pool, William dives off the board and splashes into the water. Seeing Yumi clapping, Ulrich decides he must dive too. Unfortunately, he slips off the board and slams face-down into the water. Ulrich starts sinking, but William pulls him to safety. As he drags Ulrich out of the pool, a group of kids crowd around him. Ulrich isn't breathing, but as Yumi is about to give CPR, Jim tells her to move aside and performs CPR on him, just as he regains consciousness. Yumi asks if he's okay, but Ulrich just storms off. In the lab, Jeremie virtualizes something into Lyoko. In the Forest sector, a small black sphere appears. Immediately, the small blob multiplies and becomes a black puddle of blobs.
In Odd and Ulrich's room, Odd gets a call from Jeremie telling him to come down to the Factory and to bring Aelita. Ulrich refuses to go, humiliated from almost drowning. Aelita and Odd reach the Factory and Aelita asks what program he's running. Jeremie calls it "marabounta". He tells them that Marabounta is programmed to search and destroy XANA's monsters. Odd and Aelita enter Lyoko's Forest sector on a reconnaisance mission to observe Marabounta in action. They follow a Crab that seems to be in a big hurry. At the swimming pool, William and Sissi talk about their love interests. Sissi tells William that she has liked Ulrich since elementary school and simply wants to date him. Yumi comes out of the pool, telling William that she's done and they leave. On Lyoko, Odd and Aelita see the Marabounta devouring XANA's monsters. Aelita tells Jeremie that she's seen enough. Jeremie tells them to head for a Tower, but the Marabounta surround Aelita. Odd blasts a path through the Marabounta and they run for their lives. Jeremie discovers that the Marabounta are attacking Aelita because her link to XANA makes them believe Aelita is one of XANA's monsters. Jeremie tries to terminate the program, but it doesn't work. He calls Ulrich, but he's still sulking and won't pick up. He then calls Yumi, who says she will get Ulrich to come.
Yumi goes to see Ulrich in his room. Ulrich tells her to go away. Yumi tells him not to blame her for humiliating himself and slams the door. On Lyoko, Odd and Aelita find out that the Marabounta have already reached the Tower. The Marabounta surround Aelita as XANA materializes a Crab next to her. Instead of firing at her, the Crab picks her up and tosses her to safety. The Crab is then overcome by the Marabounta and destroyed. Jeremie figures that XANA knows that Aelita needs to live for him to take her memory and that was why he sacrificed one of his own monsters. The two Lyokons are then joined by Crabs, Blocks, and Roachsters and they fire at the Marabounta. Odd takes command of XANA's monsters like a combat general from this point on. The laser fire doesn't seem to be stopping the Marabounta's advance however. Yumi reaches the Factory and is virtualized, destroying a Block before someone told her what's going on. Odd scolds her for this action, embarrassing her. At school, William walks into Ulrich's room to talk to him. Ulrich gets out of his bed and calls William a problem to be destroyed. William turns the tables on him, threatening to romance Yumi if he doesn't. William walks out leaving Ulrich to reflect for a few minutes. Afterwards, he takes off to go to the Factory. Jeremie comes up with a solution to the Marabounta: he has modified one of Odd's laser arrows with a virus, which will destroy the Marabounta. The catch: it must hit the original blob. Odd rides on top of a Crab to get to the location of the original blob. Yumi becomes engulfed by the Marabounta and is devirtualized. Ulrich, however, enters the fray and starts attacking the Marabounta. Meanwhile, the Marabounta are slowly spreading up the legs of the Crab Odd is riding on. Odd gets off a good shot, hitting the original blob. A flash of orange light expands and in it's wake, leaves no trace of the Marabounta. The surviving monsters respectfully bow and depart.
Afterwards, Jeremie admits he was wrong from the start. As they walk past Sissi, she asks Ulrich how he feels after the pool incident. Odd asks Ulrich what happened at the pool. Immediately, Yumi and Ulrich turn around and exclaim, "Nothing!"
This episode contains examples of:
- Dead Man Writing: The notes from Franz Hopper are partially decrypted here, and Jeremie makes a program called "Marabounta" based on one of his notes to defeat XANA's minions.
- Enemy Mine: The first episode to use this example. Jeremie creates the Marabounta from notes Franz Hopper left behind in his diary that he has partially decrypyted. However, while it does work, it goes after Aelita because of XANA's virus, and so XANA takes control of every monster he has at his disposal to help the crew and Aelita survive the gooey onslaught. The second episode to use this example is the following episode "Common Interest", which deals with a more subdued variant, and which it later averts.
- "Eureka!" Moment: Aelita gives this to Jeremie.Aelita: You couldn't have known my virus would be a problem.
Jeremie: Virus? Virus! - Even Evil Has Standards: For as evil as he is and how much he wants to take over the world and steal Aelita's memories, XANA actually leaves the Lyoko Warriors alone once the Marabounta is destroyed and allows them to leave to return home.
- Feud Episode: Between Ulrich and everyone else. He gets depressed at Yumi's interest in William, and gets increasingly annoyed enough to try high-diving, but fails, and nearly drowns. William saves him, and Jim resuscitates him.
- Formula-Breaking Episode: One of two episodes where XANA isn't attacking the gang, and is helping them save Lyoko from the Marabounta.
- Freeze-Frame Bonus: Overlapping with Funny Background Event, Herve can be seen smiling behind Sissi when Jim gives Ulrich mouth-to-mouth.
- Heroic Sacrifice: A Krab saves Aelita before succumbing to the Marabounta, something that throws all of them for a loop.
- Horse of a Different Color: Odd rides a Krab to get at the Ground Zero sphere that's duplicating the Marabounta, and shoots it with a killing arrow Jeremie has programmed that'll make it explode and the Marabounta explode.
- I Need to Go Iron My Dog: Odd uses this to get away from helping on Jeremie's Marabounta project.Odd: No thank you...I've got a skateboard competition.
- Mrs. Exposition: Mrs. Hertz talks about Ant life, and by shocking coincidence, Franz Hopper has similar research that can be used to defeat XANA.
- Nice Job Breaking It, Hero: Jeremie realizes this on his own when the Marabounta starts to attack Aelita. He spends the next few minutes of the episode either typing in panic or beating himself up about it.
- No Antagonist: Played straight for once. XANA isn't attacking the gang, and is actively helping them keep the Marabounta away from Aelita. He's even grateful enough to let them live to fight another day by letting them all go back to the real world.
- Screw This, I'm Outta Here: Aelita, Yumi and Ulrich all have excuses to leave Jeremie at the factory; Aelita has Italian homework, while Yumi is going to the swimming pool, and Ulrich is shadowing her.
- The End of the World as We Know It: From XANA's viewpoint, the Marabounta can essentially kill all life of Lyoko.
