A student named Milly seeks the comfort of her teddy bear after being embarrassed and ostracized due to her young age by Sissi, the school's bully/diva. Milly believes that her teddy is the only one that she can turn to for comfort and support. A corrupted super computer known as X.A.N.A. enters the real world by possessing the bear. In a quest to strike and destroy his enemies, the X.A.N.A infected teddy bear increases to an enormous size. No one is safe as this monstrosity creates a path of destruction.

Odd decides to start a music band with his friends. But Jeremy can't play, so they try to hire a drummer. In the end, the best drummer they can find is Nicholas (Sissi's blond-haired crony). As the group argues over whether or not to let Nicholas join the band (Ulrich and Yumi: Pro-Nicholas. Odd: Anti-Nicholas, Pro-Naomi), Jeremie alerts them that XANA is planning to overload a pylon and send this massive electric current through the telephone wires to blow up a nuclear power plant. Just in the nick of time, Aelita, Ulrich, and Odd stop XANA's plan. At the end of the episode, Odd gives in and allows Nicholas to be the drummer of his rock band.

Ulrich and Jeremy are doing graffiti on a wall on campus while Kiwi wakes up Jim. When Jim passes Ulrich and Jeremy, Ulrich gets away, but Jeremy just stays there and gets I'm trouble and has to stay in the campus over vacation. Sissy is being suspicious of Jeremy and thinks that he wanted to get in trouble. She asks her father to stay over the campus over her vacation, while she sorta spies on Jeremy. Jim is telling Jeremy to wash off the graffiti, then leaves while Jeremy gives Sissy the sponge and tells her (while mimicking Jim) to wash off the paint and later once again gets in trouble with Jeremy. Meanwhile, Xana has attacked again, this time spilling two toxic chemicals creating hazardous smoke. Sissy claims that she saw the smoke with arms and bumps into Jim who later finds the smoke after not believing in Jeremy and go into a small room where there is no vent and if they try to open the door, they can die. How will Yumi, Aelita, and Odd save the world and how will Ulrick find

Sissi has found something very special and dear to Ulrich, his diary! Unfortunately the diary contains some information on X.A.N.A. and Lyoko. Sissi decides to use this as a means of blackmail to get with Ulrich. Later Odd, Ulrich, Sissi, and some of the other students are going to a pool, the bus that their riding in is X.A.N.A.'s next target. He wants to ram the bus into a chemical plant. Now the team must head to Lyoko to help Aelita get to the tower to save the students and the rest of the city.

The day starts off when two girls are talking about Ulrich and then Sissi goes and interrupts and tells the girls that Ulrich is hiding his affections for her. Later, Odd, and Ulrich pull a prank on Sissi, giving her a txt message about a ""true love"". While the prank is being set, people in the town are experiencing difficulties due to a strange virus, now 2 trains are going on a collision course and one of the trains has highly toxic chemicals which can kill the town's population. Yumi cannot contact Ulrich and Odd because they shut off their mobiles. The prank is being set and Sissi opens a shed which happens to be Kiwi (Odd's dog)in there and jump om Sissy. Then Od gets in trouble after bringing Kiwi on the campus. Yumi finds the 2 and try to go to the factory, but Odd is stopped by Jim and is carried over to the principal. Now to help Aelida fix Xana, they will need everyone in the group, where is Odd when you need him?

Jeremie is working on the materialization program, and finally gets it. Jeremie wants to matierialize Aelita, but realizes that preparations need to be made. That night, Xana possesses two bulldozers and starts to destroy the factory. Odd and Yumi go into Lyoko where she falls into a virtual black hole. In order to save Yumi's life, Jeremie has to choose between either materializing her or Aelita.

This episode starts out as Yumi and Aelita are running inside Lyoko looking for the activated tower. Then Yumi suddenly disappears! She comes out of Lyoko exhausted, and looks just fine! Until Yumi Comes into Jeremy's room and takes off Jeremy's glasses and deeply gazes into his eyes and Odd and Urick come in the room and they are shocked. Then Jeremy realizes that Yumi is being controlled by Xana! Then Odd, Ulrich and supposedly Yumi.... get digitized. Yumi breaks out and trys to destroy the factory. Until they find the ""real"" Yumi in an orb and then they destroy the bad Yumi and the show ends.

When a director comes to town to film a movie, XANA posseses a plastic alien model in order to destroy Ulrich and the others.

When all the student's cell phone goes off in class, it starts Sissy on a petition to save her cell phone--but it's goes awry when Odd breaks into the school's office's locker to get his, Ulrich, & Yumi's cell phone. It turns out that X.A.N.A. is trying to get out board a highly powerful miltiary satellite in space. X.A.N.A.'s target turns out be our heroes.

Jeremie mysteriously loses contact with Aelita. The same day, a new student named Taelia (who looks alot like Aelita) arrives at school. Jeremie is convinced that Taelia is Aelita devirtualized, but the others aren't too sure. Meanwhile, XANA posseses an ancient samurai armor that Yumi brought for a project and goes out to kill Jeremie and the others. Jeremie heads to the lab, taking Taelia with him, hoping that she'll get over her ""amnesia"" and remember that she is Aelita. But what happens when Jeremie discovers that he wrong?

Jeremie learns that he has to take a test. If he passes, he'll get transfered to a new school and nobody will be able to stop XANA. Speaking of which, XANA takes control of a pack of rats. The rats attack the school and makes it nearly impossible to move around. Ulrich, Yumi, and Odd get Jim, Sissi, and everyone else to the roof and then proceed to the lab. Ulrich and Odd are virtualized, while Yumi controls the supercomputer. Does the team stand a chance with Jeremie temporarily gone?

Ulrich is fussing over a mysterious letter found slipped under his door one night... a love letter signed by Yumi! Meanwhile, XANA infects a hive of bees and is using it to attack our heroes. Ulrich struggles with his feelings and how to tell Yumi how he feels about her; that is, if he can figure it out himself, and saving the day from XANA in an episode of life and death.

It's the anniversary of the day Jeremie and the others met Aelita. They begin planning a surprise for her, unaware that Sissi is spying. When Jeremie and Odd try to open the lab elevator, it won't work. They go to the control room to see what's wrong and XANA uses the machinery in the room to attack them. They escape and reach the lab basement. Meanwhile, Ulrich and Yumi get to the elevator. This time it opens. Sissi gets in with them at the last moment, but just then XANA causes the elevator to plummet down the chute. Yumi escapes to go join Jeremy and Odd in the lab. Back at the elevator, water leaks from the walls and begins to drown Ulrich and Sissi. Ulrich promises that if they get out alive, he'll be nicer to Sissi. Aelita manages to input Code Lyoko and reset time just Ulrich and Sissi begin to drown. At the end, Jeremie and the others present Aelita with an anniversary cake and Ulrich keeps his the promise he made to Sissi in the elevator.

A new rock song Odd downloads sends him into a coma, and the same song is passed through the school. XANA's new beat is the cause, and he can broadcast it anywhere. The hospital is full to bursting with cases, and Odd's heart gets weaker with every beat it makes. But when the tune starts playing in the factory, will the crew learn that music can do more than just soothe the soul? And, will Aelita be able to save the crew? It's all up to a leap of faith...

Jeremie is now taking his ""Materalize Aelita!"" campaign to levels unknown - quite literally. He virtualizes himself, but there isn't enough memory in the system to complete the transfer, and Jeremie is caught between worlds. It will take more memory than the factory has to complete the transfer back to Earth, and if the crew doesn't hurry, Jeremie will be deleted as an old file! Could this be the final frontier for Jeremie? Will Yumi be able to pick up the slack?

Both Herb & Jeremy are in a robot competition Herb with ""Iron Sissy"", and Jeremy with ""Kiwi 2."" Herb cheats by launching a gear from Sissy to destroy ""Kiwi 2."" Meanwhile, Xana's latest plan, using the factory to built a robot. Ulrich & Yumi get help from Herb in slow down Xana's robot. Jeremy discover that Xana robot had built in the factory and messed with computer that goes to Lyoko. Jeremy fixes the super computer, and Odd helps out Alieta in Lyoko. After the time reboot, Kiwi 2 dodges Herb's gear to win the contest.

Xana decided to get rid of everyone by using the power of Gravity, and Yumi, Odd, and Jeremy went to the factory, but Ulrich had a soccer game, and his Dad was there, and as Odd said: If you're not first, then you're not good in his book. Ulrich had to save Mrs. Hertz though, because she was in the science lab. So Yumi and Odd went to Lyoko, but a huge swarm of monsters reached them, and Yumi and Odd had to clear a path for Aelita, and Yumi created a rock shield with her Telekenesis, but Yumi's Telekenisis didn't last very long, so Aelita decided to go across another way. She ended up getting hit and she fell off the bridge she was materializing. Odd and Yumi were in disbelief. But, Aelita appeared right behind them. Ulrich slammed through the science lab window, trying to save his dad, leaving Sissy with Mrs. Hertz.

When Aelita's dreams and hallucinations become too disruptive to ignore, the group heads to the Hermitage to discover their source. There, Aelita finds a doll she inexplicably recognizes to be Mister Pück. Hidden on this doll is a key to a train station locker, which contains the heavily encrypted diary of Franz Hopper. Upon its discovery, Xana takes action, possessing Jeremie, to ensure they'll never be able to read it. When they deactivate the tower and Jeremie comes to his senses, Yumi explains to Jeremie why Xana can possess him and not Ulrich, Aelita, Odd, or Yumi.

When the school holds a concert in the gym, William accidentally drops a mixing board for the DJ contest. Odd offers to fix it, and tries to get Jeremie to help. Jeremie is busy, so Aelita offers her help. She easily fixes the device, and finds that she is quite talented in using it. The school is holding its finals, as well, and Odd finds that he is not prepared in the least. Meanwhile, Xana possesses Jim and takes Aelita to the factory in order to deliver her to the Scyphozoa.

During a mission to The Carthage Region, the Scyphozoa attacks Yumi, stealing her human DNA, having her trapped on Lyoko. To make matters worse, it's picture day for her class and she will be suspended if absent. After some discussion, Ulrich gets the idea to have Sissi stand in for Yumi. Strangely, Sissi agrees and asks for nothing in return. During this, Jeremie tries to come up with a way to bring Yumi back. Aelita thinks of a way, but won't tell Jeremie about it. She only tells him once she gets to Lyoko, so he can't object. She begins to transfer her own genetic code to Yumi, which will let Xana take her memory at will, being exposed. Luckily, Odd and Ulrich get there in time to stop her. Meanwhile, Jermie figures out that Yumi's DNA should be stored in XANA's memory, which is accessible through Sector Five.

After deciphering a part of Franz Hopper's diary, Jeremie gets an idea to make a monster that fights on the side of good. By combining techniques, he makes the Marabounta, a sphere that can replicate itself endlessly and devour any monster it can encase. Though it works quite well to begin with, everything goes wrong when it senses Aelita's virus, which is linked to Xana. After Odd attacks it to save her, the Marabounta goes haywire and threatens to devour all of Lyoko, so XANA sends his monsters to help destroy the Marabounta in order to protect Aelita and her memories. The monsters leave the Lyoko Warriors without causing any trouble.

When the supercomputer's uranium power source begins to fail, Xana is forced to possess a local criminal named Peter Duncan to replace it. The supercomputer's periodic shutdowns threaten not only Xana, but also Aelita, whose heart stops beating with each shutdown.

After a Xana attack, Jeremie insists on a return trip, regardless of the fact that Xana didn't even cause any damage. When questioned, he gets defensive and angry, insisting that it's necessary. Further questioning on the matter leads to the same result. After the next return trip, Jeremie is found unconscious in his room. Yumi and Ulrich go searching through Jeremie's room to see what they can find about Jeremie's problem. Upon decoding a video diary that is stored on his computer, they learn that he had been using the return trips to increase his own intelligence. When he's about to be taken to the hospital, Xana attacks, possessing the ambulance and driving it towards the river with Yumi with him. Now the others have to save Jeremie and deactivate the tower without Jeremie's help.

Ulrich and William get into a fight over Yumi. When Xana activates a tower and materializes Krabbes, he attacks Yumi's house and destroys two scanners. While Ulrich is in detention, Yumi defends her family, and Odd distracts the Krabbe at the factory, Aelita has to make her way to the tower without any support.

After Xana possesses the principal of the school and kidnaps Odd and Yumi, he gives Jeremie an ultimatum: deliver Aelita to the Scyphozoa or sacrifice Odd and Yumi. However, Jeremie and Aelita go to Sissi for help and Sissi agrees to be tempoarily part of the group to help save her father and eradicate Xana.