The episode begins with Jeremie working on Aelita's materialization program in the middle of the night. He almost completes it, but at the last moment, it doesn't work. He starts checking to make sure that Aelita connected all the data correctly. He finds out that she defragmented the internal matrix, which he had already done, and realizes it bugged up after receiving the same information twice. He ends up yelling at her in the heat of the moment and, afterwards, tries apologizing to her. However, she brushes it off and quickly severes the link. Jeremie is left feeling guilty about his harsh words to her.
The next morning, Yumi comes to school to find Jeremie in a very bad mood. After telling her, Ulrich, and Odd about his argument, he makes the decision to go to Lyoko and apologize to Aelita in person. He asks Yumi to virtualize him on Lyoko, which she is reluctant to do. He eventually convinces her and tries to explain the virtualization process to her. To try to help ease her nerves, Jeremie gives her his notes - which is a humongous collection. Once inside, Ulrich and Odd bow and step aside, after Odd says in a British voice "Your scanner is waiting, sir." A problem occurs with the virtualization process, and Jeremie is stuck in the Virtual Limbo, basically inside the wires connecting the lab's interface computer with the Supercomputer below.
Yumi, who believes she is partially to blame for Jeremie's predicament, goes back to Kadic from The Factory to make sure the teachers don't get suspicious. Meanwhile, Ulrich is left in charge of the computer whilst Odd plays Tetris on his Game Boy.
Ulrich informs Aelita of Jeremie's problem, and, ever anxious about him, she agrees to help. According to Jeremie's theories, if she collects enough data from four towers in the four known sectors of Lyoko, Jeremie will be able to come back to the scanners. She can communicate with Jeremie due to the fact their minds have melded inside the digital world.
X.A.N.A. sees the opportunity to trap one of the most important people of the Lyoko Warriors forever in the Virtual Limbo. After Aelita recovers the data from the Ice, Desert, and Forest Sectors, it finally realizes where Aelita needs to go next, and sends Bloks to guard the last tower in the Mountain Sector. Ulrich wants to send Odd into Lyoko to help Aelita, but cannot control the Supercomputer well enough to do so. Yumi, due to her absence during most of the day, is taken to Mr. Delmas's office by Jim, who caught her trying to get out of the school.
Ulrich returns to Kadic to try and help Yumi, but cannot talk to Mr. Delmas directly, so he tries going to Sissi, who sees the chance of getting rid of Yumi for good. Even after Ulrich promises to date her for a month, Sissi refuses to talk to her father to get Yumi out of trouble. Only after Ulrich gives her a "down payment" (a kiss on the lips) does she agree to get Yumi free of the teachers' wrath.
After she is out of trouble, Yumi virtualizes Ulrich to the Mountain Sector to help Aelita. Ulrich destroys the Bloks and Aelita is able to get all the Data back and is granted access to the Virtual Limbo. There, she wakes Jeremie up and touches his fingers, thus bringing him back to the Factory. There he is knocked out and found by the group, who take him to his room. When he wakes up, he claims that his experience was "fantastic". The celebration is cut short when Ulrich has to meet Sissi according to his promise. Odd makes a remark about his "down payment", making Yumi suspicious. Fearing Yumi's reaction, Ulrich literally runs out of the room as Yumi shouts after him for answers.
Later that night, Jeremie and Aelita happily make up. She asks him when two people are in love then if that means they fight frequently. He tells her that while you do not fight all the time, it does happen. Aelita remarks how he should tell Ulrich and Yumi that, but he flatly says he'd rather "stay out of that one".
Outside of the school, Ulrich is talking to Yumi on his cell phone and desperately trying to convince her that there is nothing going on between him and Sissi. However, she refuses to believe him and angrily hangs up as Ulrich still tries to talk to her.
- The original French name for this episode is Frontière.
- This is the first episode in which X.A.N.A. does not attack Earth.
- This is also the first episode where a return to the past was not launched.
- This episode marks Jeremie's first (albeit failed) virtualization.
- The Virtual Limbo is never seen again after this episode, though it is later rementioned in Exploration and then again in the Evolution episode, Cortex.
- At the beginning of the episode when everyone is laughing at a sleeping Odd during class, Yumi can be seen sitting at one of the desks in the classroom, but she is a grade higher than Jeremie, Odd, and Ulrich.
- In one scene when Odd, Ulrich, and Yumi wait for Jeremie to come back from the Virtual Limbo in the scanner room, Odd's stomach is shown like Yumi's. Then his undershirt reappeared after Jeremie came back.
- When Jeremie wakes up in his room and Ulrich is attempting to leave, things from Ulrich's room appear next to the Einstein portrait
- When Aelita told Ulrich and Odd that Jeremie was deleting himself his life points were shown counting backwards starting at 30 going down but in the next shot his life points were counting backwards starting from 30 again.
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