The episode begins with Jeremie using the Supercomputer's Voice Synthesizer to convince Mr. Delmas he is William's father, and tells him that William has gone overseas to account for his absence from school. The next day, the Lyoko Warriors relax in the courtyard, and Jeremie and Aelita reveal that they have been using data sent to them by Franz Hopper to recreate Lyoko. That evening, the group goes to the Factory, where they succeed in recreating Sector Five but Jeremie tells them that he's not done yet as he still needs to recreate the other four surface sectors as soon as possible. Yumi and Aelita convince him that he deserves some good night's sleep as they need all the power they need to find William. After they leave the Factory for the night, William somehow returns to Earth in a scanner.
Meanwhile, Sissi, who learned of the existence of Paco, The King Of Disco after eavesdropping on Odd yesterday, appoints herself editor-in-chief of Kadic Academy's newspaper and runs a story on the film since Milly and Tamiya are on bad terms after their quarrels from last week.
The next day, when the others find William on campus, he claims he does not remember what happened after Lyoko was destroyed and that he simply woke up in the scanner room the night before. While the others are relieved William is safe, Jeremie is curious about how the latter survived Lyoko's destruction and did not fall into the Digital Sea. Later that evening, Sissi, who is looking to start more drama under the guise of reporting for the paper, pesters Aelita in her dorm about her (fabricated) backstory with Milly and Tamiya before William arrives and demands she leaves. Once he and Aelita are alone, it is revealed that William is still under X.A.N.A.'s control. He knocks Aelita unconscious and brings her to the Factory, where he virtualizes both of them onto Sector Five.
Meanwhile, Jeremie learns about William's visit to Aelita's room from Sissi and begins to suspect foul play while Milly and Tamiya make up and abandon Sissi. Jeremie notifies Ulrich and Yumi to meet at the Factory, but cannot reach Odd. It is then revealed that Jim, who is convinced that Odd is responsible for the news story on Paco, The King Of Disco, is forcing him to learn the dances performed in the film under the threat that he will move him to a different class.
On Lyoko, Aelita wakes up and manages to escape from William. Ulrich and Jeremie arrive at the Factory, and Jeremie sends Ulrich to Lyoko, where he confronts William. But before they could even fight, William demonstrates his new Super Smoke ability and chases after Aelita, leaving Ulrich alone to battle several Creepers instead. Yumi then arrives at the Factory and is sent to Lyoko as well. Jeremie warns Aelita that William is chasing her and tells her that apparently William is under X.A.N.A.'s control, even though the tower in Sector Five has not been activated.
Meanwhile, it is revealed that many Kadic students actually loved Paco, The King Of Disco, and arrive at Odd's lesson to show their appreciation for Jim. While Jim is distracted, Odd notices Ulrich's missed calls and slips away to the Factory. In Sector Five, Aelita is cornered and recaptured by William as Yumi arrives to help Ulrich take care of the Creepers. In the Celestial Dome, William and Aelita board a Black Manta, where he prepares to leave the Sector Five.
Yumi and Ulrich arrive at the Celestial Dome and give chase on the Overwing and Overbike. Ulrich tells Jeremie that apparently William is going to leave Sector Five and Yumi starts asking him if he managed to recreate the surface sectors. Jeremie denies and says that outside Sector Five there's nothing except the Digital Sea and he finally realizes that William's plan is to throw Aelita into it.
Meanwhile, Odd finally reaches the Factory and is immediately sent to Lyoko. The Black Manta releases its mines and Ulrich is devirtualized in an explosion. William, Aelita, and Yumi then exit Sector Five with Odd in tow, entering a vast expense above the Digital Sea where the Desert Sector used to be.
William then fires an Energy Slash with his Zweihänder at Yumi. She manages to dodge his attack, but ends up falling off the Overwing in the process, forcing Odd to devirtualize her before she hits the Digital Sea. William then attempts to dump Aelita, but Odd manages to catch her on his Overboard before destroying the Black Manta. Odd and Aelita return to Sector Five as William, his plan foiled, retreats back into the internet. Yumi wonders if William has been deleted but Jeremie denies that as William has returned to his new master.
Sometime later, the Lyoko Warriors discuss William's fate and X.A.N.A.'s new strategy of trying to throw Aelita into the Digital Sea by force. They then attend a concert in the gym where Jim recreates scenes from Paco, The King Of Disco.
- The original French name for this episode is Renaissance, which translates to Rebirth in English.
- This was the last episode to feature Laurent Turner as a writer.
- Consequently, this is also the only episode of Season 4 that was written by him.
- William's new X.A.N.A.-themed avatar makes its official debut in this episode, since the previous episode kept the design hidden through a dark aura.
- After Ulrich is virtualized onto Lyoko and is standing in the Arena, he asks Jeremie if he needs to trip the Key Mechanism. Jeremie responds to him saying that when he and Aelita recreated Sector Five, they got rid of the Sector Five Security Countdown process. Therefore, as of this episode onwards, the Lyoko Warriors can pass through Sector Five without having to stop the countdown.
- This episode marks the first appearance of the Black Manta.
- This is the last episode to feature the characters' original Lyoko avatars. Starting with the next episode and onwards, Aelita, Odd, Ulrich, and Yumi receive updated avatars.
- Sissi finds out that William is back while she is interviewing Ulrich in the school yard, the same time as everyone else. In the next scene, however, she seems surprised that he is back.
- In one shot when William is explaining to his friends that he doesn't recall anything after Lyoko's destruction, as he taps his head his right sleeve is black instead of red.
- In one shot when Aelita is running through Sector Five and being pursued by William, the markings under her eyes disappear.
- Jeremie could have just materialized Aelita back to Earth.
|Code Lyoko: Season 4|
|William Returns | Double Take | Opening Act | Wreck Room | Skidbladnir | Maiden Voyage | Crash Course | Replika | I'd Rather Not Talk About It | Hot Shower | The Lake | Lost At Sea | Lab Rat | Bragging Rights | Dog Day Afternoon | A Lack of Goodwill | Distant Memory | Hard Luck | Guided Missile | Kadic Bombshell | Canine Conundrum | A Space Oddity | Cousins Once Removed | Music To Soothe the Savage Beast | Wrong Exposure | Bad Connection | Cold Sweat | Down to Earth | Fight to the Finish | Echoes|
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