The episode begins with Aelita having an intense nightmare in which she and her parents are on vacation in a log cabin. A wolf appears and chases Aelita through the snow until men in black arrive and abduct her mother.
The next morning, Aelita recounts her dream to her friends as it is revealed to be winter break at Kadic Academy. Ulrich, Odd, and Yumi resent having to spend the holidays with their dysfunctional families, while Jeremie worries about leaving Aelita, who has no known family, alone during the break. Aelita, however, insists she will be okay, and one by one, the others leave.
On the deserted campus, Aelita reflects on the memories of her parents until she receives a mysterious message on her desktop. The message is supposedly from her father, asking her to come to Lyoko to see him. Curious and hopeful, Aelita travels to the Factory, where her father sends her coordinates on the lab interface. Meanwhile, on their car rides home, Jeremie continues to worry about Aelita as his father quickly understands that Jeremie likes her and Ulrich is berated by his cold and belligerent father.
Aelita virtualizes herself onto the Ice Sector using the coordinates, and finds a large Simulation Bubble nearby. Aelita enters the simulation, and finds herself at the log cabin from her dream. She enters the cabin to find Franz Hopper in the living room. After an emotional reunion, the two sit and talk with one another. Meanwhile, Yumi calls Aelita from her house to check up on her, but, being on Lyoko, she does not answer.
Suspicious, Yumi calls to inform Jeremie of Aelita's apparent disappearance. Jeremie tries to tell his father that he needs to return to campus to get a supposedly forgotten laptop, but his dad proves to be incredibly understanding and immediately turns back to campus. He then relates to knowing how Jeremie feels towards Aelita; he felt the same way about his wife, Jeremie's mother, at his age. After being called and debriefed by Jeremie, Ulrich abandons his parent's car, which is stuck in a traffic jam, and walks back to campus. Ulrich then calls Odd, who is at an airport, and informs him of the situation. Odd intentionally misses his flight and returns to campus as well.
As Aelita talks with her father, she slowly realizes the simulation is a trap set by X.A.N.A., and when she attempts to escape, William arrives and restrains her. Meanwhile, Yumi returns to campus and searches Aelita's dorm room until she finds Franz Hopper's message on her desktop. Yumi then meets up with Jeremie, who has made it back to campus, and together they rush to the Factory. Once they confirm Aelita is on Lyoko, Yumi is sent in after her.
In the simulation, Aelita manages to escape William's grip and run out of the cabin. The fake Franz Hopper transforms into a wolf and gives chase, reenacting the scene from Aelita's nightmare. Yumi arrives at the Simulation Bubble, but is barred from entering by two Tarantulas, and a battle ensues. Meanwhile, Ulrich and Odd arrive at the Factory together and are sent to Lyoko as well.
Aelita is seemingly cornered by the wolf and William, but uses her Creativity to destroy the bubble, ending the simulation. Ulrich and Odd arrive to help Yumi against the Tarantulas as Aelita faces off with William. Once they discover the bubble is gone, the three rush to Aelita's aid, and Yumi is devirtualized by a Blok, which played the part of the wolf during the simulation.
Odd and Ulrich rush to help Aelita, but William sends the Black Manta to intercept them. Using his Super Smoke to bind her, William finally succeeds in throwing Aelita into the Digital Sea. Shocked, Ulrich and Odd both advance on William in an attempt to avenge Aelita, but stop when the real Franz Hopper emerges from the Digital Sea. A second Blok appears, and together the two monsters immediately begin attacking Franz Hopper. Odd and Ulrich move in to protect Hopper as it is revealed he has saved Aelita from the Digital Sea and brought her back to Lyoko.
Odd battles the monsters to protect Hopper and Aelita while Ulrich duels with William. However, Ulrich is defeated and devirtualized by William. Odd only manages to defeat the Manta and one of the Bloks before William devirtualizes him as well. As Hopper continues to lose life points, William moves in to finish him. However, Aelita intervenes, and stops William long enough for Hopper to escape back into the Digital Sea. His plan foiled, William devirtualizes Aelita out of anger.
In Aelita's dorm room, the Lyoko Warriors discuss X.A.N.A. and Franz Hopper. Jeremie explains that Hopper found a way to inhabit the Digital Sea without being scattered and has been in hiding for over a decade. Knowing the threat that Hopper poses as long as he is alive, X.A.N.A has therefore been trying to throw Aelita into the Digital Sea in order to force him out of hiding.
To lighten the mood, Odd suggests they hand out each others' presents. Jeremie and Aelita open their gifts and find rather strange and silly gifts from Odd. However, Ulrich and Yumi don't open their gifts and Ulrich explains that in Japan a person never opens their gift in front of the person who gave it to them (leaving him blushing as Yumi smiles at him). After opening their gifts from each other, Jeremie invites Aelita to stay with him over break. She initially declines, not wanting to be a bother, but Jeremie reveals that his parents insisted on her coming over. Aelita happily accepts before reminiscing about the short time she just spent with her own father on Lyoko.
- The original French name for this episode is Mémoire blanche, which translates to White Memory in English.
- Despite his many appearances in flashbacks, tape recordings, and dreams, this episode marks the first physical appearance of Franz Hopper in the series's present timeline.
- This episode marks the second and last time where X.A.N.A. creates a simulation bubble; the first time was in Ghost Channel.
- This episode marks the fourth appearance of the Grut Advertisement.
- The boy wearing a red coat from "Saint Valentine's Day" can be seen during the traffic scene with Ulrich and his parents. Here, he is driving a truck.
- The title card features a red tower, but X.A.N.A. did not attack Earth in this episode.
- During Aelita's flashback of virtualization with her father, she is wearing her Season 4 outfit.