The entire ninth grade class at Kadic Academy prepares for a weekend-long camping trip, supervised by Jim and Ms. Hertz. As he waits in line to board the bus, Jeremie worries about the possibility of X.A.N.A. activating a tower while they are away. He also regrets not implementing a new program he and Aelita created to strengthen the shields of the Skidbladnir, but the others reassure him that he deserves a break. It is revealed that Odd has sneaked Kiwi onto the bus in a duffel bag as Ulrich calls to say goodbye to Yumi, who is still at school and who is about to take a test. At some point during the bus ride, Jim discovers Kiwi in Odd's bag, and promises to send Odd to the principle's office once they return to school.
The bus arrives at the shore of a lake, where Ms. Hertz reveals the class will spend the weekend on a small island in the center of the lake and study its ecosystem. As the class paddles to the island on rubber dinghies, Jeremie checks his laptop and finds he does not receive a strong enough signal to monitor Lyoko from their location, causing him to worry even more. Aelita calls Yumi, informs her of the problem, and asks her to check the Supercomputer regularly during the weekend. After the call, Aelita's cell phone loses signal as well.
The students pitch tents on the island while Jeremie tries unsuccessfully to get a signal on his laptop. At Kadic, Yumi reluctantly abandons a French exam and goes to the Factory, where she discovers an activated tower in the Ice Sector. Meanwhile, a spectre emerges from power lines near the shore of the lake and enters the water, transforming into an amorphous, sludge-like monster. While Ms. Hertz gives a lesson on the island's plant life, the spectre approaches the island and attacks Nicholas Poliakoff. When a crowd forms around Nicholas, the spectre attacks again, injuring Ms. Hertz and several more students.
Jeremie, Ulrich, Odd, and Aelita meet up and devise a plan: Jeremie will find a way to contact Yumi, while the others will try to get off the island. In the Lab, Yumi then discovers the Scyphozoa has appeared in Carthage, where it begins draining the Skidbladnir's shields. Yumi tries in vein to call Jeremie for help, but is called back shortly afterward when it is revealed Jeremie has built a makeshift antenna to get a signal. Jeremie then instructs Yumi to send herself to Lyoko.
Yumi virtualizes herself onto the Ice Sector, where she is confronted by two Krabs. While Yumi battles the monsters in front of the activated tower, Odd sends Kiwi out on a dinghy into the lake, and convinces Jim to use a second dinghy to paddle out and save him. While Jim is distracted, Ulrich and Aelita leave the island in the third dinghy. The spectre attacks, and both Jim and Ulrich are knocked unconscious, but Aelita manages to get to shore, and commandeers a bicycle to return to the Factory. Meanwhile, on Lyoko, Yumi defeats the Krabs, only to be approached by William. As the two dual, Yumi manages to get through to William, and he begins to fight X.A.N.A.'s possession.
As the spectre wreaks havoc on the island, Jeremie realizes his antenna can be used as a lightning rod to absorb the monster's electricity-based attacks, and herds the panicking students toward it to keep them safe. Meanwhile, Aelita arrives at the Factory and implements the shield strengthening program, saving the Skid and destroying the Scyphozoa. In the Ice Sector, William loses his fight to regain control of his body, and he and Yumi resume their battle.
Aelita sends herself to Lyoko and devirtualizes William, saving Yumi. As the spectre's attacks begin to overwhelm the antenna, Aelita devirtualizes Yumi and enters the tower. Aelita deactivates the tower, saving the students on the island, and Yumi performs a Time Reversion in the Lab.
The return to the past sends the Lyoko Warriors back to the bus ride at the start of the field trip. Jeremie reveals that, since X.A.N.A. never repeats an attack, he is no longer reluctant to go away. Odd also reveals he did not bring Kiwi on the trip this time, opting to leave him in Yumi's care instead.
- The original French name for this episode is Le lac.
- During this episode, all characters return to their old designs from seasons one, two, and three. Why this is is entirely unknown.
- William manages to fight X.A.N.A.'s possession for the first time in this episode.
- In the English version of the episode, Yumi has a French language exam. In the original French version, she has an English language exam.
- This was the first time the Scyphozoa was destroyed.