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 researching ways to send the other Lyoko Warriors into the internet from Lyoko.
At school, elections are being held for class representative, and Jeremie is one of the candidates running against Herb Pichon and, to his surprise, Odd. Meanwhile, X.A.N.A. activates a tower in the 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 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 Jeremie, Aelita, and Odd go to the Factory. William Dunbar, who is in detention and thus not on Yumi's field trip for unknown reasons, notices them out a window and eavesdrops on their conversation. Curious, William sneaks out and begins following Ulrich.
While the others travel 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 draws the monsters away from the tower so that Aelita can deactivate it. But before Aelita can enter the tower, the Scyphozoa appears. Jeremie and Aelita are confused on why it's still here, as it already took her memory. The Scyphozoa chases down Aelita, whom tries to fight back with her energy spheres but it dodges every single attempt. In the woods, Yumi tries in vein to evade the possessed students and teachers, and is attacked by Jim.
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 is just about to attack the Scyphozoa and stop it. But it finishes just before Odd can intervene and implants something inside her. After it leaves, Odd examines Aelita whom is revealed to now under X.A.N.A.'s control. She attacks Odd and escapes, heading for a way tower. In the woods, Ulrich is now cornered, and Yumi and William launch an assault on the possessed mob. William defeat all the students by himself, but is knocked out by Ms. Hertz, and Yumi and Ulrich are defeated by Jim and Ms. Hertz.
Desite being pursued by Odd, Aelita makes it into the way tower and enters Code: X.A.N.A., which begins deleting the entire Forest Sector from Lyoko. 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 deduces that X.A.N.A. will attempt to destroy all four of Lyoko's main Sectors to prevent the group from traveling to Carthage via the Transport Orb. Meanwhile, Yumi convinces Ms. Hertz, Jim, and the other students nothing out of the ordinary happened, while Ulrich returns William, who is still unconscious, to the detention room. The next day, Odd is elects class representative.
- The original French name for this episode is Lyokô moins un.
- This episode is one of the few times 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, and assumes the entire ordeal was a nightmare.
- The Scyphozoa debuts its ability to facilitate Possession.
- X.A.N.A. possesses more people in this episode than any other, with over 15 people under his control at the same time.
- 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.
- The Forest disappearing was foreshadowed by Mrs.Hertz during their trip to the forest on earth.
|Code Lyoko: Season 3|
| Straight to Heart | Lyoko Minus One | Tidal Wave | False Lead | Aelita | The Pretender | The Secret | Temporary Insanity | Sabotage | Nobody in Particular|
Triple Trouble | Double Trouble | Final Round
|Season 1 · Season 2 · Season 3 · Season 4 · Code Lyoko Evolution|