After a two-week vacation between semesters, the Lyoko Warriors gather in the Factory, where it is revealed X.A.N.A. has not launched an attack since he escaped from the Supercomputer, a fact that Jeremie finds to be ominous. Ulrich expresses an eagerness to continue fighting X.A.N.A. and Jeremie successfully tests an updated Superscan program that allows him to track X.A.N.A. around the world through the internet. When the five friends decide to return to campus, Yumi asks to have a private conversation with Ulrich, but decides her timing is poor, and promises they will talk later.
That afternoon, Jeremie and Aelita discover the schedule they share with Ulrich is different from Odd's, which could pose a problem when organizing counterattacks against X.A.N.A.. Odd decides to see the principal about rearranging his schedule while the others go to lunch. Ulrich stops Yumi in the courtyard and asks to have their talk, but they are interrupted by William. They try to talk in the cafeteria, but are interrupted once again when Hiroki, Yumi's younger brother, arrives to introduce his new friend Johnny to her. Shortly afterward, the Superscan detects X.A.N.A.'s presence somewhere in North America, and Jeremie and Aelita leave for the Factory. When Yumi and Ulrich are interrupted a third time by Sissi, Ulrich becomes impatient and takes Yumi into the woods, where they finally get to talk.
Yumi tells Ulrich that they are going in circles in their relationship, saying how "one day they're together, the next day they're not, one day he's jealous and the next day she is" and she just wishes to finally clear up their relationship. Yumi then declares that they can be friends, but nothing more; wanting a purely platonic relationship. This choice visibly leaves Ulrich horrified, but he reluctantly agrees to it. The disagreement begins to put strain on their undefined relationship.
Meanwhile, Odd discovers his schedule was changed because Jim, the gym teacher, finds Odd disruptive and specifically requested that he be separated from his friends. Odd goes snooping in Jim's living quarters, and discovers that he was once the star of an obscure 1970's film called Paco, The King Of Disco. Odd burns a copy of the film to a DVD and attempts to use it to blackmail Jim into changing his schedule. At the Factory, Jeremie and Aelita discover a large number of monsters have congregated in a single chamber of Sector Five, and notify the others. Ulrich, Yumi, and Odd make their way to the Factory, and are sent with Aelita to the Ice Sector.
Jeremie activates the Transport Orb, sending the four of them into Sector Five. Once they enter the Core Zone, a 6-minute countdown begins, and Aelita traverses the uneven floor of the room to reach the key on the opposite wall. The countdown ends, and the group gains access to the Celestial Dome. Jeremie informs them that the chamber containing the monsters can only be accessed via a passage at Sector Five's southern pole. He calls up their vehicles, and Yumi escorts Aelita into the passage while Ulrich and Odd battle a swarm of Mantas, who enter the passage a short time later.
Aelita, Odd, Yumi, and Ulrich enter the room with the monsters, and notice they seem to be attacking "some sort of blue ball" hovering in the center on the room. Jeremie decides to research the object while the others move in to protect it from the monsters. In the ensuing battle, both Yumi and Odd are both devirtualized, and Ulrich is left to fight on his own. Jeremie then discovers that the blue sphere is the Core of Lyoko, and that destroying it will destroy the rest of Lyoko as well.
Ulrich manages to defeat all but one of the remaining monsters before being devirtualized, leaving Aelita alone. It is then revealed that Aelita programmed combat abilities for herself during the vacation, and uses an Energy Field to trade blows with the last Creeper, saving the Core, but being devirtualized in the process. Jeremie becomes panicked by Aelita's devirtualization until they realize since Aelita is no longer linked to the Supercomputer, Jeremie doesn't need the Code: Earth program to bring her back to Earth anymore.
Later that day, Odd and Ulrich discuss Ulrich's relationship with Yumi, and Ulrich advises Odd make right with Jim. Odd then leaves their dorm and goes to find Jim, who is still upset with Odd over the attempted blackmail. Odd gives Jim the DVD copy of the film and apologizes. Jim has mercy on Odd for doing the right thing and agrees to rearrange his schedule.
- The original French name for this episode is Droit au cœur.
- At the beginning of the episode, Yumi and Aelita can be seen playing Go, a popular East Asian board game that originated in China roughly 2,500 years ago.
- First appearance of Johnny.
- This is the first episode in which Hiroki attends Kadic Academy.
- This episode introduces Paco, The King Of Disco, which becomes a running gag throughout the third and fourth seasons.
- Near the end of the episode, it is revealed that Aelita no longer requires the Code: Earth program to be devirtualized since she got her memories of Earth back thanks to Franz Hopper in the previous episode, making her human again. So, as of this episode onwards, she is able to come and go like Ulrich, Odd, and Yumi can.
- It was stated by Sharon Mann about why she couldn't do the deep boy voice for Jeremie after Season 2 anymore; it was because it was hurting her throat and she even said to Moonscoop "either you let me change that or Aelita is also gonna start sounding like she's from Jersey!"
|Code Lyoko: Season 3|
|Straight to Heart | Lyoko Minus One | Tidal Wave | False Lead | Aelita | The Pretender | The Secret | Temporary Insanity | Sabotage | Nobody in Particular | Triple Trouble | Double Trouble | Final Round|
|Season 1 · Season 2 · Season 3 · Season 4 · Evolution|