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 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 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 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 X.A.N.A.'s influence. It’s an epidemic.
Ulrich rediscovers his memories through flashbacks and wanders away from Sissi aimlessly. Jeremie understands the situation and analyses the substance: he finds the nanovirus inside. X.A.N.A. wants to create a world without memory which it can control however it wants… The group, minus Ulrich, head to the Factory, passing several amnesiacs on their way.
The Scan doesn’t find a tower, the heroes explore Lyoko the proper way… In the Ice Sector, Krabs attack them, proving that there’s a tower somewhere nearby… On his end, Ulrich, guided by fleeting memories that keep harassing him, gradually finds his way to the factory…
Jeremie, yielding to anxiety, sends his friend to Lyoko, knowing he’ll be needed for something… Before he arrives, Odd and Yumi defeat a Krab. The Japanese girl leaves with Aelita in the direction of the tower. Odd stays to re-teach Ulrich how to fight… Thinking it’s a video game, the samurai enthusiastically accepts his teachings.
The two young girls are cornered by four Krabs. Aelita cuts two off with a wall of ice, while the geisha is forced into a two-on-one fight… Odd soon arrives with Ulrich for reinforcements…
Still thinking he’s in a video game, he confronts the Krabs with impeccable will to save Aelita. The fake crustaceans are destroyed one by one until only one remains, which is then taken out in a combined assault by Odd and Ulrich…
Aelita enters the tower, while Odd explains to his friend that they have to return home… Ulrich asks how while Aelita enters the Code Lyoko… Odd’s response is in time with the Return to the Past, saying they’ll get home by use of magic…
Back in the science class, Ulrich, knowing of the virus in the cleansing gel, burns all of the liquid, making Mrs. Hertz get very angry and give him detention.
- 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.