At seven in the morning, Odd's alarm clock rings, waking Ulrich and alerting him to the fact that Odd has already left their dorm room. Ulrich then goes to see Jeremie, who has been up all night trying to research ways to send the team into the internet from Lyoko, but to no avail.
At school, elections are being held for class representative, and Jeremie is one of the candidates running against Herb and, to his surprise, Odd. Meanwhile, X.A.N.A. activates a tower in Lyoko's Forest Sector. Yumi, who is on a science class field trip, is attacked when a large spectre emerges from the bus's instrument panel and possesses everyone on board but her. She manages to escape into the woods and calls Jeremie for help.
After answering Yumi's call, Jeremie, Odd, Ulrich, and Aelita abandon the election and check for an activated tower on Jeremie's laptop in the courtyard. When one is found, Ulrich decides to help Yumi in the woods while Aelita, Odd, and Jeremie go to the Factory.
William, who is in detention and thus not on Yumi's field trip for unknown reasons, notices the group out a window and eavesdrops on their conversation. Curious, William sneaks out and begins following Ulrich (since the teacher who is supposed to be keeping an eye on William has fallen asleep on his desk).
In the woods, Yumi tries in vain to evade the possessed students and teachers and is attacked by Jim. While the others head over to the Factory, Ulrich discovers and accosts William for tailing him, but reluctantly accepts his help in aiding Yumi. Meanwhile, Aelita and Odd are virtualized onto the Forest Sector, where they are confronted by four Tarantulas. In the subsequent battle, Odd manages to destroy three of them and draws the last one away from the tower so that Aelita can deactivate it. But before Aelita can enter the tower, the Scyphozoa appears from behind it. Jeremie and Aelita are confused about why it's still here, as it already took her memory. The Scyphozoa chases down Aelita, who tries to fight back with her energy fields, but it dodges every single attempt.
Ulrich and William find Yumi, and Ulrich lures the possessed people away from her while William brings her to safety. On Lyoko, the Scyphozoa corners and captures Aelita. Odd evades the Tarantula shooting at him and runs to help Aelita. He is about to attack the Scyphozoa to stop it but the monster finishes just before Odd can intervene and implants something inside her. After it leaves, Jeremie, who is even more confused now, reveals to Odd that Aelita is still alive. Odd examines Aelita, but after she knocks him away, Odd realizes that Aelita is now under X.A.N.A.'s control. She attacks Odd and uses the Tarantula to head for a way tower. In the woods, Ulrich is now cornered, and Yumi and William launch an assault on the possessed mob. William defeats all the students by himself, but is knocked out by Ms. Hertz, and Yumi and Ulrich are defeated by Jim and Ms. Hertz.
Despite being pursued by Odd, Aelita makes it into the way tower and enters Code: X.A.N.A., which begins deleting the way tower and as well as the entire Forest Sector from Lyoko, leaving Odd completely shocked. Upon doing so, X.A.N.A.'s trance is lifted from her, and she and Odd run as the ground begins to disappear behind them. The activated tower is destroyed as well, saving Ulrich, Yumi, and William. Jeremie manually devirtualizes Aelita and Odd just before they both fall into the Digital Sea.
Back in the lab, Jeremie, Odd and Aelita realize that the Forest Sector does not exist anymore, and Jeremie then deduces that X.A.N.A. will attempt to destroy all four of Lyoko's surface Sectors to prevent the group from traveling to Sector Five via the Transport Orb to protect the Heart of Lyoko. Meanwhile, Yumi convinces Ms. Hertz, Jim, and the other students that nothing out of the ordinary happened, while Ulrich returns William, who is still unconscious, to the detention room.
The next day, Odd is elected as a class representative because Jeremie decided to drop his candidacy in order to focus on finding a way to virtualize the group directly into Sector Five.
- The original French name for this episode is Lyokô moins un.
- The Forest Sector being destroyed was foreshadowed by Mrs. Hertz during their trip to the forest on Earth.
- The Scyphozoa debuts its ability to facilitate possession in this episode.
- This episode marks the first time Code X.A.N.A. is entered into a tower; it is also the first time an entire sector is deleted.
- As a result, this is the only episode in Season 3 to feature the Forest Sector.
- This is the only episode to feature the red-haired teacher that was assigned to stay with William while he was in detention.
- The Lyoko Warriors receive no damage from even a single monster attack throughout the entire episode. The Tarantulas' lasers only manage to scathe the Overboard, and are somehow unable to hit Odd or Aelita even once.
- This may have been intentional, since X.A.N.A. needed Aelita alive.
- This episode is one of the few times in which a person who is not a Lyoko Warrior is consciously involved in defeating X.A.N.A., but a return to the past is not launched. In this case, William helps Ulrich and Yumi battle a possessed class of students, defeat the entire class, then is knocked out by Ms. Hertz in the process. After William wakes up, he assumes that the entire ordeal was a nightmare.
- When Aelita and Odd are virtualized onto Lyoko, Aelita is shown to be the first one to fall to the ground. However, in the very next shot, Odd is somehow the first one of the two warriors to land on the ground.
- In one shot where Odd is climbing down from the tree to shoot down the Tarantula, the halo on the Forest Sector's Way Tower changed from white to red.
- When Ulrich is cornered by the possessed class, he says "Fifteen against one, huh? I've been through worse." when he is actually surrounded by ten people.