The episode begins with Jeremie saying that the Interface Transfer is activated, along with the Virtual Memory and etc. He also states that the scanners are ready. He then says that this time it will work, and asks Aelita if she is ready. She says yes, and then Jeremie activates the materialization program. On his screen, Aelita's wire-frame loses color to around her head, and a (+) message appears, and so the materialization starts to work, and Jeremie then goes to the scanner room to see. The Scanner opens and then the view turns to the other.
Ulrich and Yumi are practicing Pencak Silat while Odd is playing fetch with Kiwi. Yumi then asks Odd if he's seen Jeremie this morning. Odd thinks he spent all night in the laboratory working on something very important. The three Lyoko Warriors then say, "Aelita" in unison. Yumi then kicks Ulrich. Ulrich says that he thought there was a time-out (but no one ever called a time-out). Jeremie runs as fast as he can to the other and lets them guess what he did. Odd answers that it had something to do with materializing Aelita. Jeremie then informs all three that he "finally did it". He shows them the test tube. Ulrich then asks, "You materialized a test tube?" Jeremie then tells them to look inside. Odd then asks that he only materialized one hair. Jeremie says that it is a hair of Aelita's. Yumi asks how he managed to do that.
They then head to the factory. Jeremie goes on a spiel that is a bit too long for them, but he gives the vital information that the hair contains Aelita's entire genetic/computer code. Jeremie then asks if it was simple enough and if they follow. Odd says no, and then Jeremie asks if it was "No, it's not simple enough, or no, you don't follow?" Odd says, "Uh... 'no' means we take your word for it." Aelita on the screen then puts her hand on her face and giggles. Jeremie says that it is no problem and that it will become clear when Aelita is there in flesh and blood. Ulrich asks if Jeremie will materialize her at that same time. Jeremie says that the hair was just a test before he does the entire thing. Yumi says, "Listen, making a hair appear is one thing, and materializing Aelita is not the same!" Jeremie then says that if they want to spend their whole life fighting X.A.N.A., and that when Aelita is finally materialized, they can deactivate X.A.N.A. once and for all. He then reactivates the program, but the window displays (!), signifying the program does not work. Jeremie says that he planned everything so perfectly. Ulrich says that he will be ready in a few days and that he is almost there. Odd then checks his watch, surprised, and says, "Hey, come on, guys, if we don't materialize in the classroom, we're not going to last a few days!"
After class, in lunch break, Jeremie wonders what happened. Yumi asks if the hair, containing the entire gene code, gives enough for the parameters for materialization, what would have been possibly wrong in the first place. Odd says that he is only a "hair" away. Ulrich then says that with all the hairs Kiwi sheds, he could practically make him another Kiwi. Sissi then comes up and says she'll keep quiet about Kiwi if Ulrich will be nicer to her. Yumi then threatens her, squeezing the glass barely, breaking it anyway in the process. This causes a cut to form on her hand.
In the infirmary, Yolanda puts a bandage on Yumi and says the cut is not very deep. Yumi says she only barely squeezed the glass. Ulrich says that now he at least knows what to expect when she gets angry, to which she smiles. Then, Ulrich gets a phone call from Jeremie, who tells him to meet him in his room.
Once they convene in the room, Jeremie says it is a disaster, as after he had checked Aelita, he discovered that Aelita's program is all full of bugs. He says that, during the hair's materialization, he hit a button he shouldn't have pressed, and therefore damaged the Annex Program. This means that the program linking Aelita to the towers is corrupted; if she enters the Code LYOKO into the tower, she'll be reformatted and deleted. Aelita says that a program can be de-bugged, and that they can do it together. Jeremie then asks the three to make up excuses for the teachers. Ulrich and Odd walk to their room to find Kiwi barking. The window breaks after he barks. Jim sees the window broken (but not Kiwi). Jim then says to Ulrich, "Clean up your bed; there's hair all over it."
Odd then sends Kiwi to Jeremie's room. Jeremie, with the help of Aelita, is trying to fix the Annex program, but Kiwi's chewing on Jeremie's clothes. The barking is causing damage to the school floor as Sissi is walking past and screams as the building quakes again.
At the factory, X.A.N.A. has just woken up and Aelita feels pulsations. Aelita then tells Jeremie and he says "It can't be. Not now!". The scene then takes for Jim and the Principal; Jim suggests humidity caused it, but the Principal says that it didn't look very damp. Jim then suspects that there is a dog (possibly Kiwi) and runs to dorm door to dorm door, and as soon as he hit's Jeremie's dorm the wall on the right starts to crack. Jean Pierre Delmas says that they need to evacuate the building. Ulrich says "It's X.A.N.A.", and Odd says "Oh, yeah, it's him…" The two begin to discuss Kiwi's ability to sense the attack and believe X.A.N.A. is using ultrasound to weaken the school's structures. The principal then announces over the school intercom that everyone was to evacuate the school buildings immediately. Realizing that Kiwi is still in Jeremie's room, Odd leaves and Ulrich calls Yumi as he heads for the Factory.
In the dorms, Odd gets Kiwi and as he's leaving he finds Milly and Tamiya in their room (both of whom had not heard the principal's announcement) and warns them that they must leave. Together, they head for the stairs and as they are about to descend, Kiwi barks and another quake hits the building which destroys the stairs, they turn and try to head the other way but find that they are trapped as the quake had destroyed the hallway behind them. Ulrich arrives at the lab and explains the situation to Jeremie, Odd then rings Ulrich and tells him what has happened at the school and tells him to go ahead to Lyoko, another quake then hits the building causing the floor he, Milly and Tamiya are standing on to tilt and he drops his phone. Ulrich tells Jeremie that there is no other choice than to send him to Lyoko.
Yumi meanwhile is on her way to Kadic when she sees that X.A.N.A. is now attacking the city with the ultrasound system and quickens her pace. Ulrich is transferred to the Forest Sector where he and Aelita begin to make their way to the tower, not realizing that they are being followed by a group of Kankrelats. Yumi arrives at Kadic to see the school in ruins and a teary-eyed Sissi who tells her that Odd, Milly and Tamiya are still inside the building. Another quake strikes and Odd spots the next floor down and instructs the girls to slide down with him to a safer spot.
On Lyoko, Ulrich and Aelita reach a crossroad when the Kankrelats attack, striking Ulrich in the leg. Yumi arrives at the factory to that it too is cracking from X.A.N.A.'s attack. Jeremie transfers her immediately to Lyoko, meanwhile, as Ulrich and Aelita attempt to flee the monsters, they reach another crossroad and are surrounded when Yumi shows up and they manage to destroy the Kankrelats. They then run down another path and spot the tower.
In the school, Odd has gotten himself and the girls down to the lower floor and they are horrified to find the exit blocked off by rubble when another quake strikes. On Lyoko, a Krab appears, Ulrich manages to defeat it but it is immediately followed by three more forcing Ulrich, Yumi and Aelita to take refuge in a hollow tree root. Jeremie then states that he has finished the repair program and tries to run it but to no avail, at the same time the roof of the lab is crumbling around him. Aelita then makes the decision that even though she faces deletion, she must deactivate the tower. Despite, Jeremie's protests, the three move out, Ulrich manages to destroy one of the Krabs but is devirtualized by one of the other while Yumi takes Aelita towards the tower. Aelita gives Yumi a wave and heads into the tower, Yumi then manages to destroy another Krab. Knowing that Aelita has made it to the tower, Yumi spreads her arms and allows her to be devirtualized by the last Krab.
In the school, a large series of quakes has begun to strike causing rubble to fall everywhere, Odd, Milly and Tamiya huddle close as part of the building begins to sink. In the tower, Aelita ascends to the upper floor and with Jeremie begging for her to stop, she bids him goodbye and enters the Lyoko code. The quakes suddenly stop at Kadic and Jeremie, sad, launches a return to the past.
The group is in the lab, mourning Aelita, and trying to convince Jeremie that it wasn't his fault she is gone. Yumi then suddenly remembers and asks if the hair that was materialized could restore Aelita. They place it in the scanner and watch as Aelita's codes are restored and Aelita reappears on Lyoko in one of the towers. The group is overjoyed and when Aelita asks what is wrong Jeremie states that "This time we only missed, by a hair", the group then laugh as X.A.N.A.'s pulsations stir once again.
- The original French name of this episode is D'un cheveu.
- This episode is called "By a hair's breadth" on the Official US YouTube Channel for some reason.
- The phrase "just in time", can also refer to a term used in programming, abbreviated "JIT". This could just be a coincidence, but also could be an intentional detail.
- Originally, only 13 episodes were planned for Season 1, which might explain the first half of this episode seeming similar to the season finale.
- After Jeremie talks to the group and leaves them to go to the factory, Yumi's face is shown speaking. However, it is Ulrich's voice that is heard.
- When Mr. Delmas does the simulation to evacuate the buildings, Emily LeDuc and Azra Ürgüp appear both in Ms. Hertz's class and in the courtyard.
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