Aelita is having a nightmare of herself being Mister Puck, being chased by a wolf being possessed by XANA. 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. 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 Puck.
While examining Mister Puck 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 CD's. Unfortunately, it is heavily encoded and will take a long time to decode.
Back at school the next day, XANA tries to posses Odd and Ulrich, but fails. They tell Jeremie about it and leave. Moments later, the ghost takes over Jeremie, who smashes the diaries. 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 XANA'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 Blocks in the Ice Barrier sector. Jeremie defeats Odd and heads down to the Lab, where he devirtualizes Yumi. 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 Lab and fights until Jeremie takes her out. Then Odd gets up and Jeremie grabs Odd by the wrist, shocking him until Aelita deactivates the tower.
Then XANA leaves Jeremie, who announces that he made copies of the diary before XANA destroyed them, and put them on the supercomputer, where the files are safe. Yumi theorizes that Jeremie was possessed by the ghost and Odd and Ulrich weren't because their experience of fighting on Lyoko made them immune. Jeremie goes to Lyoko, but after fighting a bunch of Megatanks, (but you never actually see him) he swears off going to Lyoko, preferring the Lab. Everyone gets a good laugh out of Jeremie's clothing on Lyoko (which we also don't get to see or have much of a description of).
This episode contains examples of:
- Alpha Bitch: Sissi, as per usual. This time, she grabs Mister Puck after the gang find it in Hermitage, and refuses to give it back for a good few minutes. It says a lot about her that the most agreeable thing she does this episode is shout at Aelita for waking her up with one of her recurring nightmares.
- The Cracker: Even Jeremie, master hacker that he is, admits defeat when he says the diary encryption is too great for him to crack so easily, and says that it'd take 1 - 2 years to pull it off.
- Chain Pain: Odd tries parrying XANA!Jeremie with a metal chain. This only gets him more hurt when the possessed genius uses electricity.
- Continuity Nod:
- To the previous episode, where XANA manages to take control of a human for the first time, and does so again with Jeremie.
- To "Uncharted Territory", where Aelita is again shown to activate the scanners, and does one better than Jeremie (despite her insistence on not being as good as him at launching the program), by putting the process on a macro countdown so she too can get to the scanners in time.
- Dead Man Writing: A digital form of this. XANA destroys the original CD's, but Jeremie reveals that he backed them all up in the lab on the supercomputer on a partiton XANA cannot access.
- Epic Fail: Odd tries to poke a possessed Jeremie in the eyes, but fails because the genius wears glasses.
- Girls Love Stuffed Animals: Aelita, for reasons unknown at the moment, has a "Mister Puck" doll in Hermitage that she later uses to sleep better.
- Lightning Can Do Anything: XANA!Jeremie's Shock and Awe powers means anything will get fried fast, which Odd very nearly dies to.
- Nerd Glasses: This is what saves XANA!Jeremie from geting poked in the eyes, much to the annoyance of Odd.
- No-Sell: Odd and Ulrich are immune to XANA's ghosts, but aren't sure why. Yumi theorises that it's because they've been to Lyoko. Jeremie, the only person to never make it to Lyoko properly, is not immune to this threat, and gets used by XANA for the episode to kidnap Aelita and take her to the lab.
- Out-of-Character Alert: Jeremie pulls a Wounded Gazelle Gambit to go to the infirmary, and asks Jim to let Aelita escort him there. Jeremie subsequently kicks Aelita to knock her out, and takes her to the lab. Yumi sees this scuffle and informs the others.
- Security Blanket: The "Mister Puck" doll seems to soothe Aelita from the nightmares when she holds it; it's revealed why in later episodes.
- Tap on the Head: Yumi uses a wrench to temporarily immobilize a possessed Jeremie, which frees Aelita briefly.
- The Juggernaut: XANA!Jeremie is way too powerful for any single member of the team to stop him, throwing Odd around, and Yumi doesn't do much better later on, as while she does knock him out, it's only for a few seconds.
- The Unreveal: The only way to not get possessed by XANA's ghosts is by going to Lyoko, and Jeremie does exactly that at the end of the episode, and gets into a one-sided battle with a few mega tanks who were waiting for him. We never find out what he looks like in 3D, and no dub expands upon what he looks like either, just saying he looked silly in whatever he was wearing. note
- Violence Discretion Shot: While we do see XANA!Jeremie's foot start to kick at Aelita, she is not seen getting hit.
- Would Hit a Girl: XANA takes no pains in physically or electrically knocking Aelita out while possessing Jeremie's body.
- Wounded Gazelle Gambit: XANA!Jeremie fakes his ankle injury in P.E to skip school and take an unconscious Aelita to the lab.
