At the beginning of the episode, XANA activates a Tower and seeps through Mrs. Hertz's computer and into a pool of nanobots in the Science Building whilst she is elsewhere, putting its own nanobot inside some cleansing gel. Some time later, the eighth grade comes in for Science class, and Mrs. Hertz talks to them about nanobots. When she leads them over to the nanobot container, she wonders why the cleansing gel is in there. But before she can destroy the gel, Odd, not knowing of the virus inside the substance, uses a peice of metal to scoop some of the gel onto Ulrich's nose. The XANA nanobot enters Ulrich's body via his nasal passages.
Whilst Team Lyoko is talking in the courtyard of Kadic after their classes, Ulrich suddenly faints, and is rushed to the Infirmary. During dinner, Sissi comes to visit him, and discovers he doesn't remember a thing. She decides to use this to her advantage and tells Ulrich she is his girlfriend, which he believes due to his lack of memories. Jeremie sees them on a Park bench together, and talks to Aelita about any activated Towers. She tells him that she senses Pulsations in the Ice Barrier, so Jeremie contacts Odd and Yumi, who go to the Factory to assist the AI.
Meanwhile, the entire school is catching amnesia. Sissi faints, promptly followed by Herb, Nicholas, and dozens of other kids. On his way to notify the authorities, Mr. Delmas faints as the nanobots travel over land to infect the rest of the French populace, and soon, the world will be under XANA's influence.
Ulrich, however, feels an urge to go to the Sewers. He does so and ends up in the Factory, where Jeremie virtualizes him. In Lyoko, Ulrich relearns how to use his weapons with Odd, who, after some practice, tells him to fight some Crabs about to get past Aelita's Creativity-induced wall of ice. He does so, and Aelita deactivates the Tower, putting our heroes back to science class, where Ulrich, knowing of the virus in the cleansing gel, burns all of the liquid, making Mrs. Hertz get very angry and give him detention. When everyone laughs at Odd's wisecrack, Sissi tells them all to be silenced, clearly indicating her high-and-mighty ways and the return of her memories.
This episode contains examples of:
- Accidental Misnaming: Ulrich, while an amnesiac, accidentally calls Odd "Jeremie". Odd quickly corrects him.
- Amnesiac Hero: Ulrich, for this episode only. He gets infected with nanobots created by XANA that give him amnesia. He initially doesn't remember anything but starts to get flashes of memories later on (specifically of his friendship with Odd, his romantic feelings for Yumi, and his Lyoko duties). He even ditches Sissi (who was taking advantage of him) to find out where his mind was telling him to go as he slowly remembers Lyoko and his duty to "do something important".
- Call-Back: Ulrich, even with amnesia, briefly reminisces his interactions with Sissi, Odd, and Yumi from "Log Book", "Big Bug", "Claustrophobia", "Teddygozilla", "Swarming Attack", and "Laughing Fit". He utters the same diss he directed at Sissi in the second-former-mentioned episode.
- Handicapped Badass: Ulrich temporarily forgets how to fight in Lyoko, but once he does a refresher course with Odd, he doesn't need his memory to kick ass, if anything, he's more competent at fighting than before.
- "I Know You're in There Somewhere" Fight: All of Ulrich's friends attempt variations on this. He's briefly exploited by Sissi to get him to believe they're dating, but he doesn't really buy it as he admits to feeling no real connection with her.
- Heroic Willpower: Ulrich does this to himself as flashes of his memories of his fellow Lyoko Warriors lead him to the factory, and even to the lab when specific parts of the route to the factory come up.
- Instant Expert: Sort of. Ulrich forgets his skills under XANA's amnesia, but it's rather easy for him to regain them after Odd spars with him for what appears to be about 10 minutes to jog his muscle memory.
- Laser-Guided Amnesia: Ulrich (and most of Kadic's student body and faculty) end up being amnesiac.
- Laser-Guided Karma: Odd pushed Ulrich into the slide with the gelatin inside. Ulrich later pushes a melted candy bar onto his forehead, having sworn to get his revenge in the classroom.
- Manipulative Bitch: Sissi possibly reaches her all-time low this season where she takes advantage of the amnesiac Ulrich and tries to convince him that they're an item, plus turn him against his friends.
- Nanomachines: Nanodoctors are instrumental to XANA's plan; take control of peoples' minds by putting nanobots in them.
- Nice Job Breaking It, Hero: As mentioned below, Odd shares the blame with X.A.N.A. causing Ulrich to lose his memory, but to be fair, there was no way he could've known about it.
- Unwitting Instigator of Doom: Odd causes Ulrich to experience amnesia for the episode, though quite a lot of other students get infected too.
- You're Not My Type: Ulrich, under amnesia no less, breaks the bad news to Sissi that he doesn't feel anything romantic for her. She doesn't take it well.
