The episode begins with a pan-shot of the Desert Sector, including a strange noise and activated tower. In Odd and Ulrich's room, Kiwi in laying on Odd's bed. The lamp attached to one of the tables starts to shake and the light bulb comes out, followed by XANA being careful not to break the bulb, it sets in on the table. Kiwi starts to growl and bark at the ghost. The view changes to the hallway outside the room and you hear Kiwi bark a couple more times.
The bell rings and you see the group standing around, waiting for Yumi, who comes to them asking, "Hi, so how did the Italian test go?", to which Jeremie say he doesn't understand Italian, but Odd, being Italian, does, saying, "You're really lame in Italian, Jeremie, old buddy," in Italian. Sissi walks by them with Milly and Tamiya following her asking questions, which annoys her.
The show then goes into the Cafeteria where Odd, Jeremie, are waiting in line, with Aelita and Ulrich already sitting down at the table. Odd and Jeremie are fighting in the line about Kiwi, Jeremie saying how unintelligent Kiwi is, while Odd is sticking up for him.
The picture then changes to show a hallway, with Milly and Tamiya walking down it. They see Kiwi sitting in the hallway and they become worried that Odd will get in trouble with Jim. Tamiya then offers to take Kiwi back to Odd's room, and Milly goes into the shower room. Tamiya then walks over to Kiwi, but his eyes show the eye of XANA, and Kiwi starts growling and barking at her. While reaching down, Kiwi bites her wrist. She gets dizzy and then becomes the first zombie of the episode. She then attacks Milly in the shower, but Milly overpowers her and gets away.
Back in the cafeteria, the group is still eating their food, well, Odd still is. He says, "So are you going to eat you chocolate pudding?" to which Aelita responds, "Isn't that your forth one?". Milly then comes running in screaming that Tamiya attacked her, to which Rosa Petitjean, the cook, tries to comfort her, while Jeremie ran a scan for activated Towers on his laptop. When Jeremie finds that a tower is activated, they decide to go to the Factory, and Ulrich calls Yumi and tells her to meet them there. Yumi is shown having dinner at her house with her family when Ulrich calls. Hiroki then bothers Yumi, giving Yumi an excuse to leave the table, saying that she's going to bed. Back at the cafeteria, Rosa starts thinking Milly's serious and tells Sissi to go get her father, to which Ulrich says they'll go with her, which brings a smile to Sissi's face. When they leave the cafeteria, they immediately see Jim. Sissi tells him that there's a problem with Milly and they find out that he too is a zombie. Sissi takes off running, away from Jim and the others. Ulrich tries to get Sissi to come back, but when it doesn't work he goes after her, telling the others to meet up at the Factory. Aelita then points out the other Zombies walking towards them to eat their flesh due to zombies being hungry all the time. They then turn back into the cafeteria. Odd tells Rosa that they are surrounded by zombies and she faints. Yumi is then shown sneaking out of her house, and Sissi running down down a path to which she then finds more zombies. Ulrich finally catches up with her and fights off the zombies. Back in the cafeteria, it is being surrounded by the zombies and Milly says, "See, I wasn't dreaming!". The zombies walk away from the door and you see William talking on the phone, but whoever he was talking to didn't believe him about the zombies, while Jeremie, Odd and Aelita surround the laptop. When they try to leave to get to the Factory, William doesn't allow them. While all this is happening, Yumi manages to make it to the factory. Ulrich is then shown to be surround in bushes with Sissi, they then decide to go back to the cafeteria thinking its a safe spot. Back in the cafeteria, Odd and the others told William about the Supercomputer, Lyoko and XANA, but he doesn't believe them. When he still doesn't let them go, they go to plan B, which is Odd distracting William, while Jeremie and Aelita try to get out. When William is completely distracted, they try to get out, but it doesn't work as Herve sees them and tells William, who then pushes Jeremie down saying, "Sorry, but it's for your own good. You'll thank me for when we're safe and sound."
Back at the Factory, Yumi makes it down to the Lab, to which she finds no one. The picture then changes back to the school, to the outside gates. A couple of police officers come. They get the attention of a worker, they ask him if he's seen anything out of the usual, to which he says he hasn't and the police leave. Ulrich is then seen running with Sissi to the cafeteria still, he calls Jeremie asking them if he's made it to the Factory yet. Back at the gates the gatekeeper sees Jim and the other zombies. Ulrich and Sissi are then shown in the bushes again, but they leave again when they see all the zombies. He calls Yumi, who asks where they all are. Ulrich tell her that there's a living dead convention at the school, and that he's trying to think of a way to get Aelita to the Factory, but it'll be hard. Back in the cafeteria, Kiwi somehow gets in without anyone noticing. Odd then see's him and goes to pick him up, but while he's picking him up, Milly tells them that that's the dog that they saw before Tamiya became a zombie. Kiwi bites Odd's hand and Odd sees the Eye of XANA in Kiwi's eye's. Odd then turns into a zombie and they force him onto the ground and tie him up. Jeremie then tells William again about XANA, saying that he is possessed.
Ulrich then goes out to distract them so that Sissi can get in the cafeteria. While all the zombies go after Ulrich, Sissi goes to the cafeteria telling them to let her in, but Herve says its not her, and to prove it. Sissi then says, "Um, I uh, I have a subscription to Pretty Girl, and I still sleep with my stuff moose.", but that doesn't work and Herve still wont let her in. She then insults everyone in the cafeteria when the zombies come back and Nicholas says, "Okay, that's Sissi alright.". When Herve opens the door for Sissi, Jim pushes in and Nicholas tries to throw a chair at him and then William tries to fight him, but it doesn't work for him and he's is thrown outside to the other zombies. While most of the zombies are distracted with William and the others on Ulrich, Jeremie and Aelita escape to the Sewer in the Park. The picture then changes to view them running away from the zombies that weren't with Ulrich (William is now a zombie due to being bitten by the rest of the zombies), and Yumi pops up and saves them. Back at the cafeteria Ulrich manages to get in a close to the doors. In the Factory, Jeremie goes into the lab and tells Yumi and Aelita to go to the Scanners. He then virtualizes them into Lyoko in the Desert Sector. He then virtualizes the Overwing. Back in the cafeteria, Sissi and Milly are getting scared when the Zombies are huddled in a group. In the park, Kiwi leads the Zombies that are with him to the sewers.
In Lyoko, Yumi and Aelita find the activated tower and three Crabs guarding it. Back in the Factory, an alarm goes off on the Factory Interface and Jeremie finds out that the zombies lead by Kiwi are in the Factory. In Lyoko, the middle Crab starts to fire at them, but is destroyed when Yumi throws her fan and hits it. Then the other two start firing at them and Yumi gets hit. In the cafeteria, Ulrich leads the remaining students to arm themselves as the zombies enter, which causes Rosa to faint again. On Lyoko, they lure one of the Crabs away from the tower, and destroy it, Aelita used Yumi's fan when they were flying over it. Back at the cafeteria the students and zombies fight but Tania is pulled away by Jim and the zombified staff and students close in on her and turn her into a zombie with Herve running away. Back at Lyoko they then fly up and get shot by Kankrelat and crash down on the ground and the Overwing is devirtualised. Back at the Cafeteria, the zombie William and two other students overpower Herve and turn him into a zombie. Another student is overpowered by the zombified staff and turned into one along with Nicholas. Ulrich, Milly and Sissi are the only ones left. On Lyoko, the third Crab starts to chase them. Yumi then decides to stay behind and fight the Crab while Aelita goes to the tower. She then destroys the Crab by jumping on it's back and using her fan. She then catches up to Aelita. When they get out of the maze, they run into another problem, the ground and the tower have disappeared. Then Jeremie tells them that its still there and he can see it on the screen, and Aelita uses one of her powers and to see the tower. Yumi then says she will telepathically transport Aelita to the tower, which concerns Jeremie. While in the middle of transporting Aelita, the Kankrelat comes back and shoots Yumi, causing Aelita to fall, but Yumi then catches her. In the Factory, the lift opens and Kiwi and the other zombies get in. Yumi then tries to destroy the Kankrelat, but misses and drops Aelita, but when she tries to pick her back up, she gets hit and Aelita then drops to the ground of the sector, but not into the Digital Sea. Yumi is then hit one more time and gets devirtualised. Aelita then runs for the tower and XANA virtualizes another Crab. She then runs under the Crab and goes into the Tower. In the Factory, Jeremie hasn't realized yet that the Zombies are in the lab, he does though when Kiwi bites his ankle. Back in the cafeteria, things aren't going good as the students that were bitten by the zombies turned into zombies themselves and Ulrich is attacked by the zombified Odd while the zombies are now surrounding them and nearly turning Sissi and Milly into zombies. Aelita then deactivates the tower and the zombie version of Jeremie uses the Return to the Past, and time goes back to when they were standing waiting for Yumi to join them. When Sissi walks away, Ulrich and Aelita start to compliment Sissi, which confuses Milly.
The episode ends with Jeremie saying, "Yeah, in fact, you never can guess a person's true colors, can you?" with the others nodding.
This episode contains examples of:
- Cassandra Truth: When William insists on keeping everyone inside, the Lyoko-Warriors, in an effort to convince him, tell him the truth about Lyoko. Of course, he doesn't believe them.
- The Determinator: Presumably the reason Jeremie manages to use the return to the past even though he had just been zombified.
- Dramatic Irony: When Jereme tries to get away, William knocks him to the ground saying "You'll thank me for this when you're safe".
- Episode of the Dead: XANA uses Kiwi to start a zombie pandemic, with the intention to get close to Odd and infect him and the rest of the crew.
- Family-Unfriendly Violence: Averted. When the zombies converge on their victims, the camera cuts away before we see what happens next. There is also a shot where Herve's shaking hand is sticking out from behind a cart as he is zombified (a crunching sound can be heard).
- Glowing Eyelights of Undeath: The zombies have faint glowing white eyes.
- Hidden Depths: Both William and Sissi show those during the attack; William acts responsible and tries to protect everybody (which ends up being an obstacle to the Lyoko-Warriors sadly), and Sissi tries to comfort and protect Milly.
- Loophole Abuse: Despite the Lyoko-Warriors being Immune to Mind Control, XANA is able to take over Odd by zombifying him rather than using Spectres. He later does the same to Jeremie.
- Master of Illusion: XANA turns a part of Lyoko invisible; only Jeremie and Aelita can see the portion of Lyoko, but Yumi cannot. Amusingly, the reverse is true too, the Krabs cannot see Aelita, and the Kankrelats cannot see the ground, and one of the Kankrelats bumps into the tower after Aelita enters it.
- Near-Villain Victory: This is one of the few times where XANA came dangerously close to defeating the Lyoko-Warriors, to the point that it required the use of Return to the Past. Considering using the program causes XANA's power to grow, he still achieved a small victory.
- Police Are Useless: Well, this time they do at least show up, but the only person they meet is the school gardener who has been shut in his greenhouse all day and ignorant of the zombies. As such, the cops dismiss William's call as a prank and leave mere moments before the horde comes out to convert the gardener.
- Running Gag: The cook, Rosa, keeps fainting whenever Zombies are seen or mentioned.
- Technically-Living Zombie: Despite their corpse-like appearances, the zombies are very much alive, as shown by everyone who was zombified being fully alive after the Return To The Past (which doesn't bring the dead back to life).
- Thinly-Veiled Dub Country Change: Milly and Tamiya want Sissi's opinion on transfer students going from this school to France, despite the fact the show takes place in France, and in Season One's "Satellite", it explicitly shows Jeremie the map of France when XANA attacks with a Kill Sat. This is one of the few times the dub slips up, as it otherwise keeps everything else ambiguous country-wise.
- Unwitting Instigator of Doom: In trying to keep everyone safe, William actually made the situation worse by preventing the Warriors from getting to the Factory, allowing XANA's zombie horde to grow to the point that it nearly overwhelmed everyone.
- Zombie Apocalypse: XANA's attack in this episode. It's a small-scale zombie apocalypse though, as it takes place entirely in or near the school.
- Zombie Gait: How the zombies move in the episode, naturally.
