While being handed back papers in Mr. Fumet's class, Jeremie does not do as well as usual, and Ulrich receives the worst grade in the class. Meanwhile, in the principal's office, Yumi is reprimanded for frequently being tardy and absent from class, and is threatened with expulsion if she truants again.
That night, the Lyoko Warriors convene at the Factory, where they discuss the toll fighting X.A.N.A. is putting on their personal lives. Aelita suggests recruiting another person into the group and proposes asking William, but Yumi still refuses to trust him because he's "too much of a hothead." Jeremie advises they talk in the morning and the others leave the Factory. Jeremie, however, stays up late working in the lab again to try and find a way to virtualize the group into Sector Five directly and falls asleep at the keyboard. The Superscan signals that a tower has been activated, but Jeremie sleeps through the alert and only wakes up in time to be attacked by a spectre.
The next morning, Jeremie returns to campus and announces that he has found a way to virtualize Aelita, Odd, Ulrich, and Yumi onto Sector Five, and tries in vain to get them to truant and come to the Factory. Unbeknownst to everyone, the Jeremie at school is a polymorphic clone, and the real Jeremie was thrown into the Seine River. Meanwhile, the real Jeremie manages to swim to safety and returns to the Factory.
In Ms. Hertz's class, Odd is caught cheating on an exam, and Jeremie is asked to escort him to the principal's office. While in the hallway, Odd gets a call from the real Jeremie telling him about the clone. The clone becomes aware that Jeremie is calling Odd, forcing him to attack Odd and knock him unconscious. Meanwhile, Yumi is in a photography class, where she and William are sent into the darkroom alone. While she is in the darkroom, Jeremie calls her and explains the situation before asking her to gather Odd, Ulrich, and Aelita. William, wanting to impress Yumi, promises to cover for her and allows her to escape out a window.
Yumi finds Odd in the hall and revives him before calling Aelita, who is still in Ms. Hertz's class taking the exam. After Ulrich refuses to go with her, opting to stay to try and get a decent grade, Aelita joins Yumi and Odd, and the three of them travel to the Factory together. Meanwhile, the clone returns to the Factory and attacks the real Jeremie. In the resulting struggle, the real Jeremie is thrown down the elevator shaft, and the clone assumes his place at the interface.
Aelita, Odd, and Yumi arrive at the Factory, and the clone convinces them it is the real Jeremie. However, once they are sent to Lyoko, the clone reveals itself to them and begins tampering with Yumi and Odd's avatars. The Scyphozoa and four Kankrelats then appear, and Yumi and Odd discover the clone has made them entirely intangible. With Odd and Yumi unable to stop it, the Scyphozoa manages to captures Aelita who tries her best to defend herself and possesses her. Meanwhile, Ulrich finishes his test and sets off to the Factory.
The real Jeremie is revealed to have survived his fall and uses an emergency ladder to climb back into the lab. Ulrich arrives in the lab a short time later and begins fighting the clone while the real Jeremie debugs Odd and Yumi's avatars. Once they are tangible again, Odd and Yumi chase after Aelita, who is heading for a way tower to destroy the Mountain Sector.
Ulrich begins to lose the fight against the clone. On Lyoko, Odd stops to confront the four Kankrelats while Yumi continues to pursue Aelita, and he manages to defeat one before being devirtualized. Yumi fails to prevent Aelita from entering the way tower, and once inside, Aelita inputs the Code X.A.N.A. and the Mountain Sector beings to vanish, and Jeremie manually devirtualizes Yumi and Aelita before they can fall into the Digital Sea. The activated tower vanishes as well, causing the Jeremie clone to disappear.
The Lyoko Warriors reconvene in the lab where they mourn the loss of the Mountain Sector to which Aelita adds that now X.A.N.A. will be able to destroy The Heart of Lyoko without any trouble. But then Jeremie looks at the results of the calculations he made earlier and announces that he really has found a way to send them all into directly Sector Five. Yumi finally admits the group needs another member and agrees to ask William. Later that day, she explains Lyoko, X.A.N.A., and the Supercomputer to him, and he agrees to join.
- The original French name for this episode is Surmenage, which translates to Overwork in English.
- This episode and the one preceding it, Triple Trouble, are very similarly named in the English version of the series. It is unknown whether or not this was intentional.
- Near the beginning of the episode, William confesses to Yumi that he is having recurring dreams related to the events of The Secret, making this the only known instance where the return to the past program is not entirely effective for unknown reasons.
- The code Jeremie uses to get to the lab is 167.
- This is the first polymorphic clone of Jeremie that has appeared since X.A.N.A.'s Kiss. Apparently, the behavior of this version is far more convincing, as even Aelita was fooled this time around.
- While taking his exam, Odd is initially shown holding a pen. Yet in the next shot, he is shown holding a pencil.
- In one shot when the bugged laser arrows hit the Kankrelats, the halo on the Mountain Sector's Way Tower changed from white to red.
- While Yumi and Odd's avatars are bugged, Yumi is hit with a laser from a Kankrelat, despite it having just been established that she and Odd were intangible and immune to laser fire.