In the courtyard, Odd abandons his and Ulrich's foosball game in order to spend time with Camille, a new girl he is dating. Later, all five Lyoko Warriors convene in Jeremie's dorm room, where they discuss X.A.N.A.'s recent activity. Jeremie announces that he will be giving Ulrich, Odd, and Yumi lessons on how to operate the Supercomputer interface so that they may use it in the event himself or Aelita is incapacitated. Aelita, however, convinces Jeremie to let her skip the lessons in order to compete in the final round of a DJ contest she entered some time previously to potentially open for the Subdigitals.
In the Lab, Ulrich successfully sends Odd to the Ice Sector as part of Jeremie's lesson. Suddenly, however, the Superscan detects an activated tower in the Mountain Sector, and before Jeremie can send Ulrich and Yumi to Lyoko, a polymorphic spectre emerges from wiring in the ceiling and takes the form of a man in a suit. The spectre attacks and knocks Jeremie unconscious before quickly retreating from the Lab. Still unsure how to use the interface, Yumi and Ulrich call Aelita, who is in the middle of her audition and does not notice her phone ringing. Yumi then deduces that the Polymorphic spectre is on its way to attack Aelita, and leaves the Factory to find and protect her.
As Ulrich tries in vein to find a way tower for Odd to travel to the correct Sector, three Bloks appear, and Odd is forced to fight. Eventually, Ulrich finds a way to send Odd the Overbike, and Odd uses it to escape the Bloks and begin searching for a way tower on his own. Meanwhile, at the audition, Aelita finally notices her missed call, and leaves before the Subdigitals announce the winner. As she walks through town toward the Factory, the spectre approaches her and, recognizing the danger, Aelita runs away. Yumi arrives a short time later and knocks the spectre into traffic, temporarily incapacitating it.
While fiddling with the interface keyboard, Ulrich accidentally starts a special countdown. He calls Aelita, who is on her way to the Factory with Yumi, and she informs him the countdown is a delayed virtualization into the Mountain Sector. Considering it a stroke of luck, Ulrich abandons the Lab and sends himself to Lyoko. Yumi and Aelita then arrive at the Factory, where it is revealed the spectre has hijacked a car. The spectre attempts to run the girls over on the Factory bridge, but they avoid the car and manage to enter the elevator, returning to the Lab unharmed. Aelita uses the interface to direct Odd to a way tower before having Yumi virtualize her onto Lyoko.
In the Mountain Sector, Ulrich, Aelita, and Odd convene in front of the activated tower, finding it guarded by William and three Krabs. Meanwhile, the spectre enters the Lab and begins battling with Yumi. When Yumi stops responding to them, Aelita correctly deduces that the spectre has returned, and Odd volunteers to be devirtualized in order to help Yumi.
Ulrich engages the Krabs while Odd returns to Earth and electrocutes the spectre with a live wire, saving Yumi and causing the spectre to retreat. Aelita then attempts to enter the tower, but is blocked by William. Odd takes control of the interface and manages to send Yumi to Lyoko before the spectre returns and attacks him in turn. Yumi arrives at the tower and battles with William while Ulrich defeats the Krabs. When Yumi is devirtualized, Ulrich takes her place fighting William and manages to devirtualize him as well. Aelita then enters and deactivates the tower, causing the spectre to vanish before seriously injuring Odd. Aelita, Yumi, Odd, and Ulrich then revive Jeremie.
Later, on campus, Jeremie congratulates Yumi, Odd, and Ulrich for being able to operate the Supercomputer competently, and Odd informs the others he has broken up with Camille. Aelita then reveals to her friends that the Subdigitals have called to inform her that she has been officially chosen to open for them at their next concert.
Odd then brings up the attack at the swimming pool. Aelita then asks what happened and Odd said it was similar to the attack at the gym. Jeremie then yells at Odd, telling him he wasn't suppose to bring up the gym attack.
- The original French name for this episode is Leçon de choses, which means "Lessons" in English.
- Jeremie is the only Lyoko Warrior who does not use the Supercomputer in this episode.
- Ulrich's farewell clip in Echoes is taken from this episode.
- At the beginning of the episode, Odd mentions having broken up with a girl named Melanie. This girl was the inspiration for the code name of the Skidbladnir.
- Ulrich virtualizes Odd, and inadvertently, himself onto Lyoko in this episode, marking the only two times in the original series where Ulrich virtualizes someone.
- This is also the only episode in the entire series where Odd virtualized someone.
- Throughout the entire episode, the Holomap shows that Carthage is the only Sector on Lyoko that has been recreated, but Jeremie restored all of the other four Sectors in between the events of William Returns and Double Take.
The original summary can be found here.