The episode opens with Yumi and Ulrich as spectres in the laboratory in the Amazon. The pair is cornered in a hallway by several robotic spiders, and they prepare to fight. The majority of the episode is then told through flashbacks leading up to this moment.
Several hours earlier, in gym class, Jim attempts to give a lesson on vaulting, but fails to demonstrate a proper pike vault. Herb then tries, but fails as well. When Odd begins mocking Jim and Herb, Jim demands he try the vault next. Odd performs a flawless vault, but is given detention of being disrespectful to a teacher. After class, in the campus courtyard, Ulrich, Aelita, and Jeremie tease Odd for being an braggart, and Odd, convinced of his own humility, makes a bet with Ulrich that he can go twenty-four hours without bragging. Jeremie then reveals that the Lyoko Warriors plan to return to the Amazon laboratory later that day to destroy the supercomputer.
After classes let out, the group travels to the Factory via the manhole in the park. Once there, Ulrich, Odd, Yumi, and Aelita are sent to Lyoko, board and Skid, and travel to the Forest Replika. Aelita docks the Skid to a tower, which Jeremie activates for his own use. Yumi and Ulrich are then translated into the laboratory while Odd and Aelita stay on the Replika to guard the Skid.
With Jeremie's guidance, Yumi and Ulrich quickly locate the room containing the supercomputer. However, the door is locked, and Ulrich destroys the control panel, making it impossible to hack into. Jeremie suggests they find a way to cut the power to the door's locking mechanism, and guides Ulrich to the generator while Yumi stays by the supercomputer. Meanwhile, on the Replika, Jeremie detects an activated tower, and Aelita and Odd disembark from the Skid to deactivate it. However, they are confronted by William and three Hornets, and a battle commences.
Ulrich reaches the generator, but doe not know how to properly disable it. While Aelita battles William and Odd battles the Hornets on the Replika, Yumi hears movement in another part of the laboratory and goes to investigate. She finds the room containing the vivarium, which has been destroyed since Odd and Aelita's last visit. It is then revealed that X.A.N.A. has possessed the robotic spiders as Yumi is attacked by three of them.
Jeremie allows Ulrich to simply smash the generator, and in doing so, Ulrich also cuts power to the supercomputer, disabling the spiders and causing the Replika to disappear. William falls into the Digital Sea, and Jeremie declares the mission a success. Suddenly, however, a backup generator kicks in, turning the supercomputer back on, reactivating the spiders, and causing the Replika to reappear. Jeremie cannot locate the backup generator, and Ulrich and Yumi are confronted by several spiders, forcing them to flee.
Jeremie instructs Aelita to deactivate the tower on the Replika to stop X.A.N.A. from possessing the spiders, which would allow Ulrich and Yumi to destroy the supercomputer unhindered. However, William returns to the Replika as well, and Odd challenges him while Aelita travels to the activated tower on the Overboard. Odd battles William as the opening scene is recreated and Yumi and Ulrich are cornered by the spiders.
Jeremie finally locates the backup generator as Yumi and Ulrich are defeated by the spiders, causing them both to return to the Skid, unconscious. Odd offers to return to the laboratory to finish the mission, and Jeremie complies. However, Odd leaves the Skid unguarded, and two Tarantulas appear and begin attacking the ship. As Jeremie tries in vein to rouse Ulrich and Yumi, Odd arrives at the backup generator and finds it heavily guarded by spiders.
Before Aelita makes it to the activated tower, she is attacked by another swarm of Hornets and is devirtualized, forcing Odd the battle the spiders. In an extraordinary performance, Odd defeats all the spiders and manages to blow up the backup generator, destroying the supercomputer in the laboratory and completing the mission. Jeremie brings Odd back to the Skid, and he pilots the ship out of the disintegrating Replika and back to Lyoko.
The next morning, Jeremie recounts Odd's performance to Ulrich in the shower room. Odd then arrives and immediately begins bragging. When Ulrich reminds him of their bet, Odd forfeits, finally admitting "yes, I'm a braggart, and I love it that way".
- The original French name for this episode is Arachnophobie, which translates to Arachnophobia.
- The icon representing William on the interface map is blue in this episode. However, in all previous and subsequent episodes, his icon is red like those representing monsters.
- When William first arrives on the Forest replica with the Hornets, Odd refers to them as "four Hornets and William the lowlife". However there are clearly accompanied by only three Hornets.
- Near the end of the episode, Jeremie removes his earpiece as a sign of relief. However, in the very next shot, the earpiece is clearly still in his ear.