In the Science Building, a hornet flies into an open window and lands on a microscope, and a spectre possesses it. The computer in the room detects an anomaly in the hornet's body, but nobody is in the room, so nobody notices. The possessed hornet then flies out to its nest and spreads X.A.N.A.'s new means of attack to his fellow insects and infecting the entire colony.
Meanwhile, in Ulrich and Odd's dorm, Ulrich is reading something when he hears someone run up to his door and slip a paper inside, then runs away. Ulrich takes the paper, finding a letter from Yumi saying she loves him. He then goes and meets with the rest of the gang, and asks Yumi if she's okay. Yumi says yes, then asks why, to which Ulrich says "Nothing. Just asking.", and goes to PE class, in which Jeremie, Ulrich, and Odd are playing soccer, with Jeremie as the goalie. Ulrich isn't paying attention and is still thinking about the letter, when a ball hits him in the face, making him faint for a short time. He wakes up to see Odd, Jeremie, and Jim standing over him.
In the locker room, Odd inquires why he wasn't playing well that day, saying that even Jeremie was better that game, and Ulrich says that he got a letter from a girl, not specifying it was Yumi. Odd jokes about this, and afterwards, Jeremie asks Odd whether he really did play better than Ulrich or not.
In the cafeteria, Odd and Jeremie converse about whom might be Ulrich's secret admirer. Odd suggests Sandra, Claire, Milly, and Tamiya, but Jeremie says they're all too young. Then Odd decides it must be Emily, as he spots her in line to get food. When Yumi overhears and asks Odd whom Emily is in love with, as Odd doesn't mention Ulrich, Odd answers Jim, leading Yumi to say a comment about love being blind, only to find Jim behind her, who luckily didn't seem to hear.
Later, Ulrich is in the park, reading Yumi's letter again and walking Kiwi, when the canine is attacked by a swarm of hornets that badly injure him. After returning to the campus in a hurry, Ulrich informs Mr. Delmas about the possible hornet nest that might pose a danger for other students, who thanks him for the information and sends Mr. Roullier to exterminate the dangerous flying insects. After trying to kill the hornets with a gas, Mr. Roullier comes screaming back to the Administrative Building as the hornets attack him en masse. The sight forces other students to flee. The team happens to be present, and the four split up. As Ulrich flees, he is almost stung, but Yumi saves him, falling on top of him. They are both forced to take refuge in the Science Building.
Afterwards, the group gets back together in the infirmary, where Jeremie says he and Odd should go to the Factory as the hornets might be another X.A.N.A.'s attack whilst Ulrich and Yumi try to destroy the hornets on Earth. After Yumi leaves the infirmary, Emily arrives and she angrily confronts Odd about the rumor regarding her being in love with Jim. She then slaps Odd for making up the rumor.
In the sewer, Jeremie and Odd encounter the hornets that made the nests on the ladder. The angered insects fly towards them to kill them but they escape by jumping into the sewer water as Jeremie points out that the insects are sensitive to the various smells. Once they return to Kadic, Jeremie decides to make a robot with ultrasounds to keep the hornets out of the way. Ulrich finally talks to Yumi about her letter and tells her that he feels the same way. She says she didn't write the letter, resulting in the hysterical laughter of Sissi, Herb, and Nicholas in the background. Then Jeremie and Odd return, and inform the two they must go to Lyoko as soon as possible. They finally get to the Factory with the help of Jeremie's Hornet Control Robot, which is capable of creating a high-pitched sound that repels the hornets but as Ulrich is climbing on the ladder the batteries run out and Ulrich barely avoids the hornets. Jeremie then virtualizes Yumi, Odd, and Ulrich into Forest Sector, where Aelita is waiting.
Meanwhile, Sissi is talking to Herb and Nicholas about how her genius plan, the letter, worked out. Herb and Nicholas suddenly look at something, drop their books, and begin to run screaming towards the cafeteria. Sissi turns to see what they were so frightened about, only to observe the gigantic swarm of hornets coming straight towards her. She quickly outruns her two accomplices, and actually makes it to the cafeteria before them.
On Lyoko, Odd and Yumi valiantly protect Aelita against a group of Hornets while Ulrich justs stands there doing nothing, infuriating Jeremie, who sees Odd and Yumi losing life points rapidly. Odd and Yumi have 80 life points. Odd takes a chance and runs to a Hornet. It sprays poison, giving him 30 life points and is hit again with the poison but before being devirtualized, he manages to destroy it. After Yumi tells Ulrich to snap out of it and stop thinking about what Sissi did to him, she and Aelita head for the tower while Ulrich holds off the Hornet. Yumi has only 20 life points left and tries to destroy a Hornet, but fails and gets Devirtualized. Ulrich comes just in time and destroys it with his katana.
In the cafeteria, firemen have come to the aid of the children, but they are all stranded inside the eatery. One of the hornets manage to get inside, and soon, the entire swarm is inside the lunchroom. The students panic wildly, crouching under tables and such. Just as a bunch of hornets are about to kill Sissi and her cronies, Aelita types in the code in the activated Tower, and Jeremie activates a return to the past.
Once back in time, Ulrich is in his dorm like he was in the morning. Knowing Sissi would put her letter under his door, he opens it before she can run away. Yumi is there as well, waiting. Sissi blushes and says it was for someone else. Yumi, knowing it was not, tells the brat to put it under Herb or Nicholas's door, saying they (especially Herb) would be delighted.
- The original French name of this episode is Attaque en piqué.
- This episode marks the first production appearance of Jeremie's Hornet Control Robot.
- This episode first aired on May 4th, 2004 in Columbia.
- This episode was included in X.A.N.A. Possessed.
- This is the second and final episode in which Hornets use their poison attack.
- After Ulrich reads the letter, the schoolyard is shown. And Ulrich and Odd can be seen facing each other, but in the next scene where Ulrich and Odd are present, Ulrich arrives and meets the rest of the group, including Odd.
- Jeremie told Yumi that she had 20 life points remaining after a laser strike from a Hornet at the beginning of the fight on Lyoko. However, after being hit by another laser and being damaged by a physical hit-and-run from another Hornet, Jeremie told Yumi that she still had only 20 life points.
- When Ulrich, Yumi and Odd were fighting the three Hornets, Yumi and Odd defeated two of them, but when Ulrich went to fight, three Hornets were featured instead of one.
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