At the beginning of the episode, X.A.N.A. 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 notices how it hasn't been cleaned properly and comments on the gel inside it. She has Ulrich take out a scoop for classroom examination and Odd, not knowing of the virus inside the substance, shoves Ulrich's nose into the sample as a joke. The X.A.N.A. nanobot enters Ulrich's body via his nasal passages.
Whilst Team Lyoko is talking in the courtyard of Kadic after their classes, Ulrich suddenly has an intense head ache and faints. He is promptly rushed to the Infirmary by Jim. During dinner, Sissi comes to visit him and discovers he doesn't remember a thing - having amnesia. She uses this to her advantage and tells Ulrich that she is his girlfriend and that Odd, Jeremie and Yumi are his worst enemies - all of which he believes due to his lack of memories.
Outside, the rest of Team Lyoko see the two together and are confused about why he is with Sissi. Odd comes up to him and asks how he is doing. Believing Sissi's earlier words, he snaps at Odd to stay away from him before Sissi drags him off. Jeremie comments on how Ulrich doesn't seem to remember them. Jeremie and Yumi leave to see what the nurse has to say about Ulrich's condition while Odd stays behind to see if he could bring help out Ulrich.
He watches from afar as Ulrich and Sissi are sitting on a bench in the park where Sissi tries to flirt with him. He then witnesses Ulrich reject Sissi and tells her that she simply isn't his type. He finds this amusing until Ulrich suddenly notices him. Despite Odd insisting they are actually good friends, Ulrich angrily tells him to leave. Odd leaves just as Ulrich has a brief series of flashbacks - all of which consist of his good memories of his friendship with Odd. Sissi tries to get him to come back with her, but Ulrich leaves to be alone.
Jeremie and Yumi go to the infirmary and see that it is full of sick students. After being kicked out of the room, Odd comes up to them and confirms that Ulrich indeed has amnesia. Jeremie notes how amnesia is not a contagious disease and has the theory that the earlier science lab has something to do with it. Going to the science lab, Jeremie examines the gel. He then sees that X.A.N.A has created nano viruses that are likely meant to attack memory cells, meaning it is not only contagious, but could lead to an epidemic. The three go to the factory to deactivate the tower, making the choice to leave Ulrich out of the mission given that he has no recollection of them or Lyoko. The three friends then rush to the factory in a hurry, seeing various students slowly losing their mental state to amnesia.
Meanwhile, Ulrich goes back to his dorm room (after asking a departing student for directions). He is greeted by Kiwi before looking around the room and stumbles upon a polaroid photo of him with Team Lyoko. He leaves the room and walks outside, seeing a string of students passed out along campus. Ulrich is then hit with another string of sudden memories - this one being various moments of his romance and feelings for Yumi.
At the factory, Jeremie talks to Aelita about any activated Towers. She tells him that she senses Pulsations in the Ice Barrier, so Jeremie sends Odd and Yumi to assist her. During this, Ulrich walks into the forest and has various flashbacks of his many trips the factory with his friends. He then slowly wanders to the factory, letting his pieces of memories guide him to it.
The heroes explore Lyoko to find the activated tower in the Ice Sector. Upon arriving, Krabs attack them, proving that there’s a tower somewhere nearby. While they battle the Krabs on Lyoko, Ulrich finally makes his way to the Supercomputer and finds Jeremie maning it. Ulrich wants answers, but Jeremie explains he doesn't have the time to tell him. However, Ulrich tells him that despite the fact that he has no memory whatsoever, he does know that he has something important to do there and wants to know what it is.
Jeremie decides to send him to Lyoko, knowing he’ll be needed for something. Before he arrives, Odd and Yumi defeat the Krab and Yumi leaves with Aelita in the direction of the tower. Odd stays behind to re-teach Ulrich how to fight on Lyoko. Under the belief that it’s all a video game, Ulrich enthusiastically accepts and the two begin to practice.
The two young girls are cornered by four Krabs. Aelita cuts two off with a wall of ice, while Yumi is forced into a two-on-one fight. Odd soon arrives with Ulrich for reinforcement, just as Yumi is devirtualized. Still thinking he’s in a video game, he confronts the Krabs with impeccable will to save Aelita when Odd tells him they have to protect her. The two successfully take out the Krabs as Aelita makes her way to the tower.
Aelita enters the tower while Odd explains to Ulrich that the mission is over and they will return home. Ulrich asks how while Aelita enters Code Lyoko. Odd’s response is in time with the Return to the Past, saying they’ll get home by "magic". The Time Reversion then takes place, saving all the Kadic students and staff.
Back in the science class earlier, Ulrich, knowing of the virus in the cleansing gel, burns all of the liquid in the lab. An angry Mrs. Hertz declares that she will give him a punishment "that he won't forget". Ulrich simply says that he "sure hopes not", leaving her confused. He and his friends then have a good laugh at the inside joke.
- The original French name of this episode is Mémoire morte.
- While sending Ulrich to Lyoko, Jeremie says that the Scanner Room is one floor up from the lab.
- During the battle Ulrich was in, the backs of both ID cards were Odd's.