The episode opens with Jeremie working from his dorm room desktop on a new program to reinforce the shields of the Skidbladnir. Stressed and irritable, Jeremie crankily turns down an offer to play foosball in the rec room with his friends. However, Mr. Delmas soon arrives in Jeremie's room, where he reveals that Patrick Belpois, Jeremie's cousin, has just enrolled at Kadic Academy, and will be sharing Jeremie's single-occupancy dorm until an available room is found for him.
After Delmas leaves, Odd, Ulrich, Yumi, and Aelita enter the dorm and meet Patrick for the first time. Patrick is greeted warmly by Jeremie's friends, and connects with them quickly. However, in an angry outburst, Jeremie demands they all leave his dorm so that he may work, and warns Patrick coldly to stay away from his friends. Despite the warning, Patrick is invited to play foosball in Jeremie's stead, and goes to the rec room with Odd, Ulrich, and Yumi. Aelita, however, stays behind and confronts Jeremie over his recent rude behavior. Jeremie then admits to having a poor relationship with Patrick, who is very sociable and often overshadowed him during their childhood. Aelita, accuses Jeremie of being jealous and petty before leaving.
In the rec room, Odd bad-mouths Jeremie for being rude to Patrick, but Patrick defends his cousin. Across the room, Sissi eavesdrops on their conversation, and formulates a plan to cause drama. Meanwhile, in his dorm, Jeremie is alerted to a new Replika, and calls Aelita, telling her to meet at the Factory. Aelita goes at the rec room and cryptically notifies the others, who abruptly abandon Patrick and their foosball game to go to the Factory with her. Patrick, confused, decides to return to his and Jeremie's dorm room alone.
As the Lyoko Warriors travel to the Factory, Jeremie reveals that he left the Skid's shield program running on his dorm computer to finish updating, and that he locked to door to keep Patrick from seeing it. Meanwhile, in the dormitory building, Patrick arrives at Jeremie's room and uses a key the principle had given him earlier to unlock the door. Sissi arrives shortly afterward, feigning kindness, and attempts to convince Patrick that Jeremie and his friends are bad people. Sissi then calls Herb to the dorm as well, and asks him to access Jeremie's personal files on his desktop, despite Patrick's objections.
After being virtualized, boarding the Skid, and departing from Lyoko, Aelita, Ulrich, Odd, and Yumi are approached by three Sharks on the way to the Replika. Aelita releases the Nav Skids, and a battle ensues. Meanwhile, in Jeremie's dorm, Herb cancels the shield program to access other parts of the computer, turning off the Skid's shield and putting Aelita in grave danger. When the Skid is hit with a torpedo from one of the monsters, Aelita loses all control of the vessel and all contact with Jeremie is lost, leaving her floating in the Digital Sea.
Jeremie investigates the shield program and discovers someone is tampering with his dorm computer. He then activated a firewall to keep Herb from accessing any more of the Supercomputer's functions before being alerted to an activated tower in the Desert Sector on Lyoko. Jeremie instructs Ulrich, Odd, and Yumi to protect the Skid while he returns to campus to correct the shield program. Meanwhile, on Jeremie's dorm computer, Herb finds an extensive archive of the group's illegal activities, including Aelita's falsified identification papers and medical history, recordings of wire-tapped phone calls, and more. Sissi plans to use this information to get Jeremie expelled for Kadic, but Patrick stands up for his cousin in protest. However, during a standoff in the dorm room, two spectres emerge from a wall outlet and possess Sissi and Herb, who proceed to attack Patrick.
In the Digital Sea, Ulrich, Yumi, and Odd abandon their battle with the Sharks to search for Aelita, who has been swept away in the tide of the Digital Sea. Meanwhile, Aelita and the unresponsive Skid are approached by the Kalamar, a large squid- or spider-like monster which uses a drill to attempt to destroy the ship. On campus, Jeremie arrives at the dormitory building to find Patrick in the hallway, kicked out of the dorm room while Herb attempts to breach Jeremie's firewall. Patrick vows to have help Jeremie, and the cousins reconcile. They then storm into the dorm room, but are overwhelmed by the superpowered Sissi.
Yumi, Ulrich, and Odd finally find Aelita, and move in to save her from the Kalamar. However, they soon find that the Nav Skids have lost a significant amount of power due to the main Skid being offline, and cannot fire torpedoes. Defenseless, the trio is forced to flee when the Sharks catch up to them. Meanwhile, when Herb succeeds in cracking the firewall, Jeremie decides to cut the power to the computer to stop him from meddling with the Skid any further. While Patrick holds Sissi off, Jeremie runs to a storage room on the ground floor and turns off the fuse box, cutting electricity to the entire building.
Herb's connection to the supercomputer now gone, the Skid is put back online, saving it from the Kalamar, and the Nav Skids have power restored to them, allowing Ulrich, Odd, and Yumi to defeat the monsters. As the Skid rushes back to Lyoko, Sissi critically injures Patrick and follows Jeremie to the storage room, where she quickly defeats him in the resulting struggle. Finally arriving in the Desert Sector, Aelita, Ulrich, Yumi, and Odd battle two Tarantulas guarding the activated tower, and Yumi is devirtualized in the process. Aelita then enters and deactivates the tower, saving Jeremie from Sissi. Jeremie then returns to Patrick's side and calls Aelita to perform a Time Reversion as Patrick slips into a coma. Aelita then performs the return to the past, saving Patrick.
The Lyoko Warriors return to Jeremie's dorm room, where they meet Patrick again. Jeremie, having learned his lesson, takes a break from working on the shield program and decides to go play foosball with the others. Jeremie then admits his childhood jealousy to Patrick, and the two reconcile before Mr. Delmas returns and informs Patrick that his parents have decided at the last minute not to enroll him in Kadic, and that he will be returning home shortly. Momentarily disappointed, the two cousins then decide to go to the rec room anyway to spend time together.
- The original French name for this episode is Cousins ennemis, which translates to Enemy Cousins.
- This is the only episode to feature Patrick Belpois. It is also the only episode in which the Kalamar appears.
- William is notably absent in this episode.
- This was the last episode to air on American television. However, the English versions of the remaining episodes were later broadcast online.
- At the end of the episode, Aelita calls Patrick's phone after Jeremie's is destroyed by Sissi. However, since Aelita has only known Patrick for a few hours, it is unknown when, where, and how she acquired his phone number.
The original summary for this episode can be found here.