Aelita is having nightmares of a small elf being chased by wolves. The wolves catch up to him as Aelita wakes up. Jeremie suggests that they go back to the Hermitage. When they get there, Aelita has another vision. Once again, the nightmare of the wolves returns. The vision guides Aelita to look behind a painting of a tree and there, she finds a small doll that looks like the elf. Even though she has seemingly never seen it before she knows it's name: Mister Pück.
While examining Mister Pück at school, Sissi comes by and teases Aelita by taking the doll and throwing it around, causing a small key to fall out. Ulrich grabs the key and gives it to Jeremie, who finds out that it is the key to a railway locker. When they look inside the locker, they find Franz Hopper's diary on a set of CDs. Unfortunately, it is heavily encrypted and will take a long time to decode.
Back at school the next day, X.A.N.A. tries to possess Odd and Ulrich, but fails on both attempts. They tell Jeremie about it and leave. Moments later, the ghost takes over Jeremie, who destroys the discs containing the diary. Later in gym class, the possessed Jeremie fake-twists his ankle. He asks if Aelita can take him to the infirmary. When they are out of sight he knocks Aelita out and kidnaps her.
As Jeremie takes Aelita to the factory, the others are informed of the situation and begin their pursuit. At the Factory, Ulrich, Yumi, and Odd fight off Jeremie and get Aelita away from him. Everyone goes down to the lab, except for Odd, who is fighting Jeremie. Unfortunately for Odd, Jeremie has X.A.N.A.'s powers, including the abilities to punch steel and the power to electrocute. Odd puts up a good fight, but it's hopeless.
On Lyoko, Yumi and Ulrich fight off four Bloks in the Ice Sector. Jeremie defeats Odd and heads down to the lab, where he devirtualizes Yumi. A Blok freezes Aelita's leg so she can't get to the tower. Ulrich is left trying to destroy the Bloks, then the Scyphozoa comes and grabs Aelita. Odd comes down to the lab to continue the fight and tries to poke Jeremie's eyes with his fingers, but he remembers that Jeremie wears glasses. Yumi, devirtualized by Jeremie, comes up to the pab and fights him until Jeremie takes her out. Ulrich cuts the Scyphozoa's tentacles and Aelita runs to the tower. Odd then gets up and Jeremie grabs Odd by the wrist, shocking him until Aelita deactivates the tower.
Jeremie, freed of X.A.N.A.'s influence, announces that he made copies of the diary before X.A.N.A. destroyed them and put them on the supercomputer, where the files are safe. Ulrich still wonders why X.A.N.A.'s ghosts were unable to control him or Odd and have no trouble possessing Jeremie. Yumi theorizes that it must be because their experience of fighting X.A.N.A.'s monsters on Lyoko made them more resistant.
At the group's suggestion, Jeremie goes to Lyoko, but after fighting a bunch of Megatanks (off-screen), he swears off going to Lyoko, preferring the lab. Everyone gets a good laugh out of Jeremie's "ridiculous" (off-screen) appearance on Lyoko. That night, Aelita is all tucked in bed and is fast asleep, holding her childhood doll. We hear a little girl's voice and fade to black.
- The original French name for this episode is the same as the English.
- This is the first episode of Code Lyoko to be aired in the US before it aired in France; the rest of Season 2 from Mister Pück onwards also aired in the US first.
- It is revealed in this episode that X.A.N.A. can't possess people who have been to Lyoko several times.
- This episode marks the first time where Bloks use their Fire Rings.
- This episode was featured in Sector 5 Mystery.
- First appearance of Mister Pück.
- In the bathroom, when Odd goes to find Ulrich, he is seen wearing his gym outfit instead of his pajamas, but then, in the next shot, Odd is wearing his pajamas again.