The episode begins with Jeremie and Aelita watching a garbled and grainy video on Jeremie's laptop in his room. One voice is heard talking to someone else in the video. Aelita asks Jeremie where he got this video from, and he says it's from the Cortex data from the last mission and that he overlooked it accidentally with all the chaos going on at the time. After listening to the video file a bit more, Aelita asks Jeremie if one of the voices could be her father. He replies that he can't confirm it for certain, but if it is, her father is extremely mad about something.
Before he can run the corrupted video file through one of his decoding programs, his watch goes off, which means that it's time for PE Class. Jeremie leaves the laptop running the decrypting program while he and Aelita run off to class.
Meanwhile, Ulrich and Odd are already doing exercises in class when Jeremie and Aelita arrive at the last second. Ulrich mentions to Odd that he saw Samantha just before class, and to this information, Odd becomes elated with excitement and starts to dance instead of doing the proper drills in class. Jim notices this and sends him off to run around the courtyard ten times before coming back. Odd runs off and fakes an injury, so Jim lets him run to the infirmary. Jeremie, who didn't hear what Ulrich had said to Odd before, asks why Odd is acting so happy today; and Ulrich informs him of Samantha's return to Kadic. Aelita jokes at this news, saying that Samantha will indeed bring some spice to life now.
Instead of running off to the infirmary, however, Odd rushes to see if Ulrich was right about seeing Samantha. He rushes off to the lockers and finds Sam, but unbeknownst to him, a spectre with the appearance of a random school-girl watched him run past and is now following him. Odd is happy that Sam's back, and tells her to meet him later after he changes clothes. After the conversation, the spectre runs around a corner and morphs into a copy of the janitor.
Back in Gym Class, Jeremie gets an alert on his phone about a X.A.N.A.'s attack and lets Ulrich and Aelita know about it. Jeremie runs off and fakes a similar injury to Odd's, and Jim excuses him from class as well, complaining what was with everyone today. Aelita tells Ulrich that she'll warn Yumi and William, and Ulrich says he will get Odd. Ulrich also tells her to fake collapsing and that he will bring her to the infirmary in order to get out of gym class as well.
In Odd's room, Odd throws his phone and other clothes on top of his bed trying to find the perfect clothes to meet with Sam.
The scene shifts to the Factory, which Jeremie has already arrived at by the time Yumi and William show up. Yumi asks where the tower is this time, and Jeremie tells her that it's in the western side of the Desert Sector and that he'll send backup when the others arrive. He virtualizes the two, and William jokes to Yumi that he's already warmed up so there's no need for reinforcements. Yumi asks how far the tower is from their present location, and Jeremie says that it's not too far away, just after the canyon area. William runs off towards the tower, and Yumi yells that it's not a race to get there.
Back in the Cafeteria, Odd (who's wearing nice clothing and also has his hair straightened and neat) sees Sam and proceeds to walk over to her. However, the polymorphic spectre trips Odd and grabs him, and it starts to steal source codes from him. Sam runs up and chastises the "janitor" for knocking Odd over, and the spectre lets Odd go. She asks if Odd is okay; but when Odd tries to speak, all that comes out of his mouth is complete gibberish. Sam wonders what wrong with him, but Odd accidentally calls her an ugly beetroot, which makes her leave in an angry mood. Odd now realizes that something is terribly wrong with him.
Back on Lyoko, Yumi and William are still making their way through the canyon when Yumi hears something out of nowhere. William says that he didn't hear anything, but soon after, a squad of Kankrelats ambush them from the top of the canyon. The two warriors soon begin battling, and William quickly destroys four while Yumi takes care of another two of them. William walks over, bragging that it seemed like an easy attack today, but one final Kankrelat shows up. Before it can hit him though, Yumi tells him to jump and she destroys the final monster. She then proceeds to joke about William's ill-timed statement before. Jeremie warns them to stay focused because there is a lot of unknown activity happening around the tower, and he tells them to wait for Ulrich and Aelita to catch up to them. Ulrich and Aelita are then vitualized, and Aelita tells Jeremie to call Odd again to let him know about the spectre. He says he will, and then he virtualizes the Overbike for them. He also notes that the spectre is somewhere around the cafeteria currently, to which Aelita responds about her hatred of those "code-eaters" and the side effects associated with them. Ulrich turns back to her and jokes about Odd needing to worry about Samantha's unwanted side effects rather than the spectre's, to which Aelita can only roll her eyes. Jeremie informs them to keep their minds on the mission and to hurry through the canyon while he calls Odd. Ulrich and Aelita then make their way to meet up with the other two. Jeremie calls Odd but gets no response.
Back in Kadic, Odd realizes that he's doesn't have his phone on him (he left it in his room), but then he spots Jim on his cell phone. He runs over to Jim, and Jim sees him and says that he's trying to call the infirmary. Odd tries to talk; but more nonsense come out, and upon hearing this, Jim tries to get in touch with the infirmary once more. He asks Odd if he's okay, but Odd's more focused on the janitor-spectre whose morphs in front of him into the school gardener. Odd immediately grabs the phone from Jim and runs away, and Jim concludes that Odd is definitely not okay.
William and Yumi are still walking through the canyon, and Yumi tells William to wait up for Ulrich and Aelita to arrive. However, William says that it'll be better for them to know what they'll be up against before the other two show up; to which Yumi retorts by saying that four people have better odds than two. William starts to question Jeremie's warning about activity around the tower because of how calm it seems, and Yumi can only comment about William's stubbornness. Walking around the corner, both of them are in shock at what they see: an 11×11 wall of Bloks is in their way. The Blok Wall fires a giant laser shot and devirtualizes William, and the blast from it throws Yumi back a great distance. Immediately after this, Yumi is devirtualized. Jeremie has been monitoring this and hectically warns Ulrich and Aelita to take cover asap. They arrive in front of the Blok Wall and stare on amazement as well. The Blok Wall fires again, but they take cover behind some boulders; however the Overbike is destroyed. Aelita remarks that she's never seen anything like that before and asks Jeremie for advice. All the advice he can give is just to hide for the moment. Jeremie then gets a call from Odd, and like Jim and Sam before him, he can't understand what Odd's trying to say. He then realizes that a spectre must've touched Odd and tells him not to move. Jeremie then turns to Yumi and William and gives them his phone and tells them to find Odd. He turns back to the two on Lyoko to find out what's happening, and Ulrich responds that the wall seems to be indestructible and that they can't move forward. Aelita suggests that she fly over the wall with her wings, but Ulrich remarks that she's not fast enough to make it over there; he also asks Jeremie for a solution to this major obstacle in their path. Jeremie tells him that's he trying his best, and at that same time, a shot from the Blok Wall destroys the boulder hiding Ulrich and Aelita. Jeremie tells Aelita to protect herself and Ulrich using her Energy Fields, and she manages to make a large shield to repel a shot. Ulrich tells Jeremie that it worked and that they'll try to move forward, but Aelita appears to be exhausted from blocking the first shot. Ulrich helps her up and keeps her motivated to hold the shield up. Another shot gets deflected into the wall beside them, and Ulrich compliment Aelita on her actions; but the wall that just got hit starts to crumble. Jeremie tries to warn them, but the wall falls on them both, devirtualizing the two.
Back in the factory, William and Ulrich fuming at how poor their performances on Lyoko were, and Jeremie also chimes in more bad news with the fact that there's a spectre on the loose and that Odd's in danger. Aelita asks Yumi how Odd is doing, and she says that she really can't tell because she can't make out what Odd's saying. Jeremie then says the top priority is to find Odd before anything else happens, so Yumi and William start to head to the elevator. He also says that he'll continue trying to find a chink in the Blok Wall's armor. Yumi asks Jeremie what to do about the spectre, and Jeremie tells her to avoid stepping on the flower beds because the spectre's form is now the gardener's.
Yumi and William make it back to school without any problems but find that it's almost impossible to locate Odd because of his inability to tell where he is. Yumi tells him to make a sign for them, and William spots someone waving an arm by the lockers. They find Odd behind one of them, and he's hurriedly trying to tell them something but nonsense comes out. Yumi and William try to calm him down, and Yumi notices Jim's cellphone in Odd's possession. He tries to explain himself, but again, jargon comes out. Yumi tells William to take Odd back to the factory while she returns Jim's phone back. Odd hectically tries to tell Yumi something over and over again, and Yumi is surprised at the damage the spectre was able to inflict this time around.
In the factory, everyone is relieved to see that Odd is safe, and Jeremie starts filling Odd in on the dire news about the Blok Wall protecting the tower. Odd goes over to the computer and attempts to point something out about the spectre, but Jeremie tells him that they'll talk later once Odd's on Lyoko. Odd is virtualized and his speech is back to normal, to everyone's delight; although, William jokes about preferring the messed up Odd more. Odd then is finally able to relate the message he was trying to tell the others before: the spectre is a polymorphic one. He tells them to warn Yumi about this, so Ulrich and William team up to find her at school. Jeremie lets Odd know that they're taking care of the situation.
Inside the gym at school, Yumi finds Jim and returns his phone to him. Surprisingly, Jim's is furious as Yumi would guess, but rather understands about Odd's condition. He puts the blame on the rigorous schedules all the students make at Kadic: too much academics and not enough extracurricular activities. Yumi agrees with him, and Jim takes the initiative to tell Mr. Delmas about this dilemma, making Yumi come with him. Unknown to both of them though, the polymorphic spectre was hiding behind some mats, and it transformed yet again from the gardener to Mr. Delmas.
On Lyoko, Jeremie programs Odd what he refers to as a "kinetic lens" in order for him to get a better advantage on defeating the Blok Wall. He states that it'll be able to analyze the structure of the Bloks from a distance to find a weakness. Odd likes the gadget, and Jeremie programs the Overboard for him as well and tells him to make his way to the canyon. He also warns him not to get too close. Meanwhile, Aelita is trying to get in touch with Yumi to warn her about the spectre.
Yumi and Jim are sitting outside the Mr. Delmas's office, and Yumi can't take Aelita's call. Jim is trying to come up with a sound argument for over-exhausting the students with homework, and the real Mr. Delmas gets the door and lets them both inside his office to talk.
Jeremie tells Odd to abandon his Overboard and to take cover from the Blok Wall. He says that he'll have to walk from now on to hide from the Bloks' sight. Odd responds that he'll use his ninja mode that Yumi taught him awhile back, and Jeremie jokes that Odd meant Master Ishiyama. Odd manages to get closer to the Bloks unseen, but then he starts to lament about what he said to Samantha when he was speaking nonsense. Jeremie notices on the screen that Odd's story about the polymorphic spectre was correct, and Aelita tells him that she'll alert William and Ulrich. She calls them up and informs them that the spectre is now taking the form of Mr. Delmas.
The guys then agree to check to see if Yumi is in the principal's office, and when they arrive they see her walk out his door. Ulrich asks if she's okay, and Yumi responds yes but also that they'll have free orange juice from now on at the ten-o'clock breaks. William believes the spectre got to her because of what she just said, but Yumi says that everything fine. Ulrich asks if Delmas ever touched her, to which Yumi responds saying they shook hands. William starts to ponder who the spectre is now.
Odd starts stealthily crawling towards the edge of the canyon to analyze the Blok Wall, and like everyone else before, he is amazed at what he sees. When he sees the Bloks rotate, he compares them to train station signs and jokes about a train that runs from Lyoko to Kadic. Jeremie tells him to cut the jokes and to focus on the mission at hand. He tells Odd that the wall probably acts like a closed circuit, and when Odd asks about that, he explains that there's a "master" Blok that's controlling all the "slave" Bloks; destroy the master Blok, and the whole wall will be destroyed. Odd bemoans at the hard task given to him and remarks that the all look the same. Jeremie tells him that the point is that they're not all the same, that by using the eye scanner he should be able to pick out the right Blok like a game he refers to as the "Seven Errors game." Odd tells him that his game stinks.
In the forest, Yumi, Ulrich, and William are heading back to the factory. Yumi asks about Odd, and William tells him that he's already on Lyoko but also states that facing the Blok Wall will not be easy. Ulrich tells William to trust Odd. They do not realize that the polymorphic spectre is following them back.
Jeremie asks Odd if he's spotted anything unusual yet, to which Odd says that he hasn't; but then Odd notices one Blok unlike the rest of the others. Odd asks where he should fire, and Jeremie asks him why he's even asking about that at all and just shoot it like any normal monster. Jeremie also informs him that everyone is in the lab now, and Odd sarcastically replies not to put any more pressure on him and fires a few Laser Arrows. The shots miss, and the Blok Wall fires back but misses Odd. Jeremie tells Odd he should use his Overboard, and Odd dodges another shot and in the process jumps upon the Overboard and fires more Laser Arrows which are accurate this time around. The master Blok is destroyed, and the other in the wall start to disappear as well. Everyone is pleased with Odd's performance, and just before entering the tower, Odd celebrates by showing off some dance moves and moonwalks into the tower, jokingly asking who the boss is. As Odd is ascending the tower though, Samantha arrives in the elevator to everyone's surprise. Odd asks if what he heard is correct, but Aelita looks at the computer screen and exclaims that that isn't Samantha, but rather the polymorphic spectre. Jeremie tells Odd to hurry up and input the code, and Odd asks what is going on. Jeremie yells at him not to worry about that and to quickly enter the code into the interface. Odd does so, and the spectre is destroyed before it can run and grab the warriors.
Back in Jeremie's room, Odd hooks up to the laptop and is sad to learn that due to the events of today, X.A.N.A. is up to 80% strength. He starts to blame himself, but Aelita steps in and reminds him that he did a great job today and that he shouldn't be so hard on himself. Odd, however, is unamused and says that today was a catastrophe because of what he did to Samantha earlier. Ulrich jokes that Odd could do like he did before and blurt out some random phrases but end it with a compliment. The group quietly laughs, but Jeremie notices that the file he and Aelita were looking at before has finally finished rendering. Everyone walks up and watches the uncorrupted video: it contains Franz Hopper yelling at another person named Tyron about his instructions being clear about plans never coming out of a "vault." Yumi confirms with Aelita that that is her father, and the video continues with Hopper accusing Tyron of using those plans for his own selfish purpose. Tyron admitted he did, but he accuses Hopper of being ambition-less and unable of seeing the bigger picture of what these plans could be used for. Ulrich turns to Aelita and asks if she knows the man her father is talking to, and she responds that she's never heard of him. The video goes on with Hopper firing Tyron from his research team, and Jeremie concludes that Hopper is not responsible for X.A.N.A.'s return, that the person who betrayed Franz is the one responsible. Before Jeremie can finish saying his name, Aelita says it: Tyron.
Both the episode and the video end with Hopper walking out and Tyron muttering to himself that Hopper is too naive.
- The original French name for this episode is Compte-à-rebours.
- First appearance of Samantha Suarez.
- First appearance of Professor Tyron.
- Tyron refers to Franz Hopper by his alias instead of his real name, Waldo Franz Schaeffer. This is strange because their interaction is during the time of the Carthage Project, and Waldo did not change his name until after he had left the project.
- X.A.N.A.'s first polymorphic spectre of Season 5 is created in this episode.
- This is the first time since X.A.N.A. Awakens, Part 1 that Odd has had his hair straightened.
- William's Energy Slash ability is demonstrated for the first time in the fifth season in this episode.
- Kankrelats make their first appearance in the fifth season in this episode.
- Bloks can now be "stacked" and be made into a wall of Bloks.
- Apparently, Yumi is skilled in a stealth technique called "ninja mode" and has taught it to Odd.
- After Yumi is devirtualized by the Blok Wall's Laser blast, the interface shows that Odd was virtualized into Lyoko instead of Ulrich.