The episode starts with the Lyoko Warriors attempting to deactivate a tower in the Mountain Sector. Ulrich battles a Tarantula, while and Odd and Yumi each fight a Krab. However, mid-battle, each of them experiences a bug that causes their avatars to freeze, giving the monsters an opportunity to devirtualize them. Aelita, as well as the monsters, encounters similar issues, but manages to make it into the tower and deactivate it.
Back in the Lab, Jeremie suggests the bugs were part of X.A.N.A.'s attack, and decides to run a few routine checks to be sure. Because boys' dorms are being inspected on campus, Odd leaves Kiwi in the Lab before he, Ulrich, Aelita, and Yumi leave the Factory. During the checks, Jeremie discovers via the Factory's security camera system that Michael Riley, the groundskeeper at Kadic Academy, had been possessed by X.A.N.A., sneaked into the Supercomputer room, and damaged several of the Supercomputer's memory cards.
The next morning at breakfast, Jeremie explains to the others what happened; Riley's damage caused a drop in the calculating capacity of the Supercomputer, resulting in the bugs they experienced. He also discovers that the damage in spreading, and that he has three hours to fix it before the Supercomputer becomes unsalvageable. However, before he can leave for the Factory, he is stopped by Jim, who demands to inspect his room. Jeremie fails the inspection because of the clutter in his room, and is forced by Jim to stay and clean it. Jeremie gives Aelita a box of new memory cards, silently asking her to fix the Supercomputer in his place. Aelita complies, and set off for the Factory.
While Jeremie cleans his room, his desktop alerts him to an activated tower in the Ice Sector, but Jim finds his cleaning job insufficient and demands he stay longer. Aelita discovers the activated tower as well when she arrives in the Lab. She notifies Ulrich and Odd, and then goes down to the Supercomputer room to replace the damaged cards, which temporarily mixes the problem.
Ulrich notifies Yumi of the tower before he and Odd set out for the Factory. However, as they travel through the park, it is revealed that X.A.N.A. has possessed the trees, and the two boys are captured by tentacle-like roots. Meanwhile, Yumi, who is walking with her class to an art museum, slips away and heads into the woods, where she too is ensnared by roots.
Jeremie finally cleans his room thoroughly and is allowed to leave. On his way to the Factory, he finds Odd and Ulrich trapped by the roots, and uses a derelict shovel to free Ulrich before being trapped himself. Ulrich manages to reach the Factory, where Aelita virtualizes him and herself onto Lyoko.
Ulrich and Aelita find the activated tower guarded by two Tarantulas. As they battle, Ulrich, Aelita, and the monsters all experience bugs, and the two Lyoko Warriors retreat. In the park, the bugs affect the trees as well, and Jeremie and Odd manage to free themselves and travel to the Factory, where Odd is virtualized onto Lyoko. Meanwhile, the roots drag Yumi to a naturally occurring fissure in the ground and attempt to pull her in.
On Lyoko, Odd's Overboard and Ulrich's Overbike disappear due to the bugs, and they and Aelita are confronted by a Megatank. In the woods, the roots succeed in pulling Yumi into the fissure, where she falls several metres to the bottom. Meanwhile, Jeremie goes down to the Supercomputer Room and finishes Aelita's repairs. However, the Supercomputer does not have enough energy left to reboot and stop the bugs. Desperate, Aelita suggests they deliberately destroy the Ice Sector, which would lessen the workload of the Supercomputer and give it enough power to reboot. Jeremie reluctantly agrees, and Aelita enters an unguarded way tower.
As the roots begin to suffocate Yumi, Aelita inputs Code: X.A.N.A., and the Ice Sector begins to disappear. Ulrich, Odd, and Aelita devirtualize one another to avoid falling into the Digital Sea, and Jeremie is able to reboot the system. The activated tower is destroyed along with the Sector, and the roots stop attacking Yumi, allowing her to climb out of the fissure.
Later that night, the five Lyoko Warriors convene in Jeremie's dorm. Ulrich tends to Yumi's wounds while Jeremie discusses the increased necessity of finding a way to virtualize the others directly onto Sector Five. Odd then accidentally knocks over Jeremie's shelf, making his room as cluttered as it was before the inspection.
- The original French name for this episode is the same as the English.
- At the end of the episode, Jeremie claims the repairs he and Aelita made to the Supercomputer were only temporary. However, the bugs never return and the problem is never mentioned again.
- This episode marks the only time in the series Aelita intentionally enters Code: X.A.N.A. into a tower.
- In the scene where Jim was first discovering the mess in Jeremie's room, Jeremie handed Aelita a box behind Jim's back. When Jim lifts the blanket on Jeremie's bed to find his computer magazines, as he is looking at the group, the box is still in Aelita's hands. However, in the next shot of the group, the box is gone and Aelita's arms are crossed. After they all leave, Aelita is holding the box again.
The original episode summary can be found here.