The episode opens with Yumi receiving a call from Jeremie in the middle of the night to go to the Factory. After sneaking out of her house, William accidentally and literally bumps into her on his skateboard, who then proceeds to flirt with and harass her. She tricks him into closing his eyes and sneaks away, leaving him confused and embarrassed. Yumi arrives at the Factory, only to find that Aelita, Odd, and Ulrich have already deactivated the tower and are returning from Lyoko, leaving Yumi even more irritated with William.
The next morning, William angrily confronts Yumi about ditching him and demands to know where she was going. Still angry herself, Yumi tells him that "she now finds his two-bit Casanova act sickening" and warns him to leave her alone. William's hostile mood then causes him to speak out of turn with Jim, who gives him detention. Meanwhile, Jeremie and Aelita are working on a history report together until they receive an alert from the Superscan that another tower has been activated in the Ice Sector. Rather than truant, Ulrich and Odd convince Jeremie and Aelita to stay and give their report while the two boys leave for the Factory.
William takes a walk in the park, where he discovers Odd and Ulrich climbing into the manhole and follows them to the Factory. Back at Kadic, Jeremie and Aelita rush through their report and ask to go to the nurse's office. Once out of class, they head for the Factory. Meanwhile, Ulrich calls Yumi's cell phone, but she is in swimming class and is unable to reach her.
As Aelita and Jeremie enter the elevator in the Factory, William, who is hiding nearby, ducks in behind them and confronts them about what they are up to. When they arrive at the lab, Jeremie suggests they simply ignore William and focus on the tower. However, when Ulrich mentions that Yumi is not answering her phone, William offers to retrieve her from the pool. The others reluctantly agree to let him help.
Odd, Ulrich and Aelita are virtualized onto Lyoko and begin to make their way toward the tower while William leaves the Factory to get Yumi. At the pool, William tactlessly shouts X.A.N.A.'s name out loud and tells Yumi to come with him. When Jim attempts to send him to the principal’s office, Yumi volunteers to take him herself, and the two use the excuse to leave for the Factory.
Meanwhile, on Lyoko, Ulrich, Aelita, and Odd are pursued by a large group of Kankrelats. Ulrich and Odd split up and defeat all the monsters, meeting up again at the tower. On the main floor of the Factory, a construction worker possessed by X.A.N.A. arrives and begins placing charges around the Factory before also setting a detonator.
Yumi and William arrive at the Factory, but are confronted by the construction worker on the bridge. Yumi calls Jeremie to inform him of the possession before the worker attacks. On Lyoko, Ulrich, Odd, and Aelita approach the tower and are confronted by two Bloks. Both Ulrich and Odd are devirtualized, but Aelita succeeds in deactivating the tower, saving Yumi and William and causing the worker to pass out.
Jeremie calls Yumi, who confirms that she and William are alright. However, William informs them that the formerly possessed man works for a company that specializes in building demolition, and correctly deduces that there are explosives hidden around the Factory. Before Jeremie can launch a return in time, Aelita exits the tower and is captured by the Scyphozoa, allowing X.A.N.A. to possess her. Yumi is sent to Lyoko to stop Aelita while Ulrich, Odd, and William search the Factory for the detonator.
Yumi begins pursuing Aelita, who is being defended by a Megatank on her to a way tower. On the Factory floor, Odd manages to wake up the "ex-X.A.N.A.-fied" construction worker, who advises they look for the detonator somewhere high up. Meanwhile, Yumi battles the Megatank in a narrow canyon. She defeats the monster, but her Overwing is also destroyed in the process, forcing her to proceed on foot to stop Aelita.
Ulrich finds the detonator in the rafters with two minutes remaining in the countdown, and William volunteers to try and diffuse it. On Lyoko, Yumi engages Aelita in front of the tower, and the two girls fight. William succeeds in short-circuiting the detonator, preventing an explosion, and Yumi and Aelita trade blows simultaneously. Yumi is devirtualized, but Aelita dodges her fan. However, as Aelita runs toward the tower, the fan flies back around and strikes Aelita, devirtualizing her, ending her possession, and saving the Ice Sector from deletion.
All six students then convene in the lab, and William asks them if he could join the Lyoko Warriors. Jeremie, Aelita, Ulrich, Odd, and Yumi agree to cast anonymous ballots, and that the results must be unanimous for William to join. However, there is a single "no", and William is asked to leave. William, upset, enters the elevator after insulting Ulrich, assuming him to be the dissenting voter, and Jeremie performs a return to the past, erasing William's memory and getting rid of the explosives that were set around the Factory.
Back in the dorms, Odd, Jeremie, Ulrich, and Aelita discuss the vote. Ulrich reveals he voted "yes", and they deduce Yumi was the actual dissenting voter. Meanwhile, Yumi looks out her bedroom window and watches William pass her house. Speaking to herself, she says "Sorry William, but I don’t think you can be trusted."
- The original French name for this episode is Le Secret.
- This episode marks the only time where a Megatank fires its laser while rolling.
- This episode also marks the only time in the season where X.A.N.A. is prevented from destroying a sector on the first try.
- When Yumi and William were hanging from the bridge, William confessed to Yumi that he cannot swim. Yet in Marabounta, he was able to save Ulrich from drowning in the pool.
- When Yumi and Aelita trade blows, Yumi throws both of her fans, but in the next shot, only one fan is in the air while the other is still in her hand.
|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 · Evolution|