The episode starts with a spectre emerging from a street lamp and possessing a crow. The desktop in Jeremie's dorm room signals a tower has been activated, but Jeremie is in the shower room and does not receive the alert. When Odd and Ulrich, who are in the shower room as well, argue over who performs better on Lyoko, Jeremie informs them that he has been keeping track of how many monsters Odd, Ulrich, Yumi, and Aelita defeat on Lyoko, and that these statistics will show who the "best" Lyoko Warrior truly is.
Ulrich is then approached by Johnny, who asks to have a private conversation. Back in Ulrich's dorm room, Johnny confesses to having a crush on Yumi and asks Ulrich for advice. Ulrich decides to haze Johnny, tricking him into pestering Yumi as much as possible to get her attention. When Ulrich tells Odd, Aelita, and Jeremie what he said to Johnny, they scold him for being petty and jealous as Yumi hates being followed around and she could tell Johnny to get lost.
Johnny takes Ulrich's advice and begins incessantly following Yumi which is getting on her nerves. Later, while Odd, Ulrich, Jeremie, and Aelita relax in the courtyard, they are attacked by a large flock of crows, all of which are now possessed by X.A.N.A.. They flee into the woods, where they enter the sewer and set off for the Factory.
Meanwhile, Yumi agrees to teach Johnny how to vault on a pommel horse in the gym until she is also attacked by the crows and is forced to take refuge in the garden shed. However, the crows enter the shed through the window and attack Yumi en masse, leaving her severely injured.
Jeremie, Odd, Aelita, and Ulrich make it to the Factory to find the bridge covered in possessed crows. They are attacked, but all manage to escape into the elevator unharmed. Jeremie then sends Aelita, Odd, and Ulrich to the Desert Sector, where they encounter several Krabs.
Back on campus, Johnny runs away and returns with Jim and Mr. Delmas, who find Yumi unconscious in the shed. Jeremie calls Yumi's cell phone and Mr. Delmas answers, informing him of Yumi's condition and that she is going to be sent to the hospital. On Lyoko, Ulrich and Odd start competing with each other to see who is better and thus they both losing lots of life points. Odd is devirtualized due to his arrogance and bragging, but Ulrich and Aelita succeed in defeating the Krabs. Aelita deactivates the tower, but before Jeremie can perform a return to the past, he is attacked by crows that managed to get into the lab and loses consciousness.
A large swarm of Hornets arrives at the now deactivated tower, and Ulrich and Aelita are forced to fight once again. However, the Scyphozoa also appears and captures Aelita. Ulrich's katana is destroyed by the Hornets, and the Scyphozoa has possessed Aelita for X.A.N.A.. Then a Manta flies down to the crater and Scyphozoa handles Aelita on it's back and then flies away along with three Hornets.
Meanwhile, Yumi wakes up in the back of an ambulance and begins talking with Johnny, who is riding with her. She discovers that it was Ulrich who told Johnny to follow her around, but loses consciousness again shortly afterward. Odd discovers Jeremie's unconscious body in the lab and takes control of the interface, trying in vain to virtualize Ulrich a new sabre.
Ulrich follows Aelita and the monsters to the way tower and tries to restrain her to prevent her from entering Code: X.A.N.A.. In the lab, Odd resorts to slapping Jeremie, waking him up and allowing him to retake control of the interface. However, despite Ulrich's best efforts, Aelita manages to enter the tower and input the code, destroying the Desert Sector. Jeremie is frustrated by the loss of another sector, but Odd reminds him that it's not too late to save Yumi, so Jeremie launches a return to the past in order to save her.
Back in the boys' shower room, Jeremie not only confirms Yumi is okay, but she is now furious knowing that Ulrich is the reason why Johnny was pestering her. So, when Johnny approaches Ulrich for advice, he tells him to say "Ulrich is a really dumb jerk, and wants to apologize for pulling such a stupid stunt." Yumi, taking note of the indirect apology, invites Johnny to study with her. Meanwhile, Jeremie tallies the statistics and announces that Aelita is the best at defeating monsters, much to Ulrich and Odd's disappointment.
- The original French name for this episode is Le prétendant.
- This was the last episode to feature Françoise Charpiat as writer.
- While the group travels in the sewer, Aelita rides on Yumi's skateboard, despite usually riding a scooter.
- X.A.N.A.'s use of birds was inspired by the Alfred Hitchcock film The Birds. Several scenes from the film were even recreated in this episode.
- Near the beginning of the episode, Odd mentions a movie in which birds attack humans; this is another reference to The Birds.
- When Johnny is in the ambulance with Yumi, he is shown wearing a bandage on his forehead, but he wasn't attacked by the crows at all.
- In one shot where Ulrich was trying to stop X.A.N.A. Aelita from entering the way tower in the Desert Sector, the tower's halo changed from white to red.