The episode opens with Odd, Aelita, Yumi, and Ulrich relaxing in the courtyard. Aelita mentions that Jeremie has been isolating himself from the others after learning how many Replikas there are, and admits to being worries about him. However, Jeremie himself soon arrives in a happy mood, and reveals to the others that he has found a way to destroy X.A.N.A. for good. After reviewing Franz Hopper's notes countless times, Jeremie has decided to create his own "multi agent system", the same type of program as the Marabounta, that he will design to seek out and destroy all traces of X.A.N.A. on the internet. Jeremie also reveals that he has developed a program that, if executed just as he is devirtualized, could free William from X.A.N.A.'s control and bring him back to Earth.
Excited, the Lyoko Warriors decide to skip lunch and travel to the Factory to try and save William. Meanwhile, William's parents have come to visit Kadic Academy, and the William Clone is sent to greet them. However, Mr. and Mrs. Dunbar quickly become suspicious of the Clone, whose personality is strikingly different from their son's. In the Digital Sea, Aelita pilots the Skid to the Ice Replika. Once there, Aelita docks the Skid to a tower before Jeremie sends her and Odd to the Siberian research base. Jeremie then notices X.A.N.A. has begun gathering energy from the other Replikas, and becomes worried.
On campus, Mr. and Mrs. Dunbar eventually become convinced that the Clone is not their real son, and visit the principle's office to demand answers. Over the course of their argument, Jeremie's multiple lies about William being ill and on vacation with his parents unravel as the real Mr. and Mrs. Dunbar deny ever taking William overseas. On the Replika, William launched an assault on the Skid with a swarm of Mantas, and Ulrich and Yumi move in to confront him. Meanwhile, at the Russian base, Odd and Aelita discover a massive legion of robots. As Jeremie hacks a locked door for them, he postulates that X.A.N.A. plans to use the robots to enslave humanity.
As Ulrich clashes with William and Yumi battles the Mantas, the legion of robots are activated, and begin advancing on Odd and Aelita. The pair manages to hold them off long enough for Jeremie to unlock the door, and together they flee deeper into the base, searching of the supercomputer. Suddenly, William retreats in the middle of his fight with Ulrich as X.A.N.A. uses the gathered energy to unleash the Kolossus onto the Replika. Ulrich then manages to climb onto the Kolossus's arm in an attempt to reach William, who is now perched on the monster's shoulder. As Yumi and Ulrich discuss how to defeat the giant, Jeremie decides to launch his incomplete multi agent program, which he uses to immobilize the Kolossus.
At the base, Aelita and Odd reach the supercomputer, which is behind another locked door, barring their way and forcing them to defend against the robots once again. After Jeremie unlocks the door, Odd volunteers to hold the robots off while Aelita uses a control terminal to input Jeremie's program for freeing William. However, Odd is soon overwhelmed by the robots and is defeated, returning unconscious to the Skid. Meanwhile, as Ulrich reaches the Kolossus's shoulder, he and William begin battling once again while Yumi circles the Kolossus on her Overwing, waiting for Jeremie's signal to devirtualize William.
As Aelita finishes entering the program, the Kolossus frees itself from Jeremie's multi agent system and manages to devirtualize Yumi. The robots then arrive at the control terminal, forcing Aelita to defend against them. During their sword fight, Ulrich and William fall off the Kolossus and onto the ground, where Ulrich manages to subdue William. Despite the robots, Aelita succeeds in initiating the program to free William, but, on the Replika, William uses his Super Smoke to escape from Ulrich, who is then stepped on and devirtualized by the Kolossus.
Aelita is then defeated by the robots and sent back to the Skid, unconscious. As the Kolossus approaches the Skid tower, Odd is revealed to have woken up and disembarked from the ship as he sneaks up behind William and devirtualizes him. Jeremie's program is a success, and William returns to Earth, being greeted by Yumi and Ulrich in the Scanner Room. However, the Kolossus reaches the Skid, and Aelita wakes up too late to save the ship as the monster attacks. The Skid is ripped from the tower and slides across the ground before blowing up, devirtualizing Aelita just before it falls into the Digital Sea and is destroyed.
Later, on campus, William enters the dormitory building and switches places with the Clone. Jeremie then deactivates the tower controlling the Clone, causing it to vanish as the real William convinces his parents he has "not been himself", but that he is back to normal. Later, as William talks with his parents, the Lyoko Warriors convene in the courtyard and discuss the events of the day. William is revealed to not fully understand what happened to him, and Jeremie is confident he will be able to live a normal life. However, despite the fact that X.A.N.A. has lost William as an ally, the Lyoko Warriors have lost the Skid. No longer able to travel to Replikas, Jeremie puts all his faith into his multi agent system.
- The original French name for this episode is Retour, which translates to Return.
- Both the English and French episode titles are references to William's return to Earth.
- This episode marked the only appearance of the Siberian Robots. It is also the only time in the series that William's mother is seen.
- In the episode Replika, Jeremie states that when a person is devirtualized on a Replika, the Skid acts as a proxy, and allows them to safely return to Earth. In other words, the Skid is necessary to return to Earth from a Replika. However, at the end of this episode, Odd is still on the Replika after the Skid is destroyed, and thus should not have been able to return to Earth.
- After Aelita is devirtualized, when William says "I did a pretty good job, didn't I", his right sleeve is missing.