The episode opens in the scanner room, where William steps into a scanner and Jeremie creates his Lyoko avatar, officially making William a Lyoko Warrior and rendering him immune to the return trips to the past. Afterward, Yumi, Aelita, Ulrich, and Odd remind William of how dangerous X.A.N.A. is and warn him not to get overzealous.
The next day, Odd participates in an Inter-Academy Skateboard Championship where he meets with Sam, an old girlfriend from a different school, and mutual romantic feelings quickly resurface. Meanwhile, in Jeremie's dorm room, Jeremie and Aelita complete a program that will allow them to better search for Franz Hopper. When Jeremie is stopped by Milly and Tamiya, who are looking to interview him as the class valedictorian, Aelita decides to go to the Factory alone to test the program.
After implementing the program in the lab, Aelita discovers that X.A.N.A. has launched an attack on the Core of Lyoko, and notifies the others, most of whom are occupied and cannot get to the Factory: Jeremie is the subject of an argument between Milly and Tamiya, Ulrich is forced to talk about his poor grades when his father arrives at school, and Yumi is babysitting Hiroki. Desperate, Ulrich asks Sam to give Odd a cryptic message telling him to go to the Factory, and she agrees, but she withholds the information so that she can spend more time with him. William is the only available member of the group and Aelita is forced to take him with her onto Lyoko.
Jeremie manages to escape Milly and Tamiya's interview and heads over to the Factory. Meanwhile, in Sector Five, Aelita and William enter the Core Zone, where William, enchanted by the interior of Sector Five battles several Creepers while Aelita traverses the room and touches the Key Mechanism. Not taking their mission seriously, William prefers to destroy some more Creepers insteady of following Aelita to the elevator and becomes separated from Aelita in the Core Zone when it begins to reconfigure. Jeremie arrives in the lab as the Scyphozoa appears in the core zone. William thinks it is just another monster to deal with and ignores Jeremie's commands to run away from it, which leads to William getting captured by the Scyphozoa. Aelita is unable to make it back into the Core Zone in time to save him, and he is possessed. After a short battle, William devirtualizes Aelita and travels to the Great Hall.
Ulrich and Yumi manage to slip away from their responsibilities and escape to the Factory. When Odd discovers he is needed on Lyoko, and that Sam did not give him Ulrich's message, he abandons her and the Championship for the Factory as well. All three arrive in the scanner room as William and many Creepers begin to attack the Core, destroying its first shield layer in several seconds. Meanwhile, in the lab, the program Aelita implemented earlier finds Franz Hopper on Lyoko, and Jeremie rushes to rematerialize him.
Ulrich, Odd, and Yumi face off with William in the Great Hall as the second and final shield layer is destroyed. William, aided by new powers from X.A.N.A., defeats Odd, then Ulrich, and finally Yumi, all in rapid succession. Then, with nothing in his way to stop him now, William uses his levitation to get close to the Core and stab it with his Zweihänder, destroying Lyoko. Jeremie is unable to rematerialize Franz Hopper in time, and he and William are lost when Lyoko is annihilated. It is then revealed (to the audience, not the characters) that X.A.N.A. has kept William alive to be his right hand.
Later, at the Hermitage, the Lyoko Warriors discuss how they now have no way of fighting X.A.N.A., and how the entire world is unaware of and defenseless against the threat. Aelita laments the loss of her father, while Yumi wonders whether or not William is still alive. Suddenly, Jeremie receives a message on his laptop from Franz Hopper, indicating that he is in fact still alive, surviving somewhere on the internet.
- The original French name of this episode is Dernier round.
- The title card for this episode features the Holomap with Sector Five being the only remaining sector; the Forest, Desert, Ice, and Mountain Sectors were all destroyed in Lyoko Minus One, The Pretender, Sabotage, and Double Trouble, respectively.
- This episode marks the second and final appearance of Samantha Knight; her first appearance was in Rock Bottom?.
- Jeremie succeeds in virtualizing the team directly onto Sector Five for the first time in this episode. Despite the fact that it has taken him the entire season to discover how to do it, the accomplishment is not mentioned even once in the entire episode.
- This is the only episode in which William has a "good" avatar. After this, his avatar is replaced with an "evil", X.A.N.A.-themed one for the remainder of the series.
- This episode marks the first appearance of X.A.N.A. William.
- This was the last episode to have a French-language novel adaptation.
- This episode features a deleted scene in which William stabs Yumi in the chest and the Zweihänder's blade sticks from her chest for a few seconds until she collapses.