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 snuck 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 principal's office as soon as 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 its 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. 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, the situation gets even worse as Yumi discovers that the Scyphozoa has appeared in Sector Five's North Pole, where it begins draining the Skidbladnir's shields. Yumi tries in vain to call Jeremie for help, but is called back shortly afterward when it is revealed that Jeremie and Aelita has built a makeshift antenna to get a signal. Jeremie then instructs Yumi to strenghten the energy in shields in order to protect the Skid but she can't handle with typing the complicated commands. He then tells to send herself to Lyoko to secure the tower.
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 takes Sissi's bicycle, which was left on the main land, to travel to the Factory. Meanwhile, on Lyoko, Yumi defeats the Krabs but one of their lasers knocks her inside the tower. Yumi waits there only to be ambushed by William, who was waiting inside. He kicks her from the tower and the two beign fighting. As the two duel, William has the upper hand and almost devirtualizes Yumi, but she pleads "No William!, you're a Lyoko Warrior, you swore it!", this makes William to remember when he joined the group 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 activates several towers in each surface sector to implement the shield, which strengthens the program, saves the Skid, and destroys the Scyphozoa in the process. In the Ice Sector, Yumi tries to help William, but he 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 launches a return to the past in the lab.
The return in time 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, who is shown to be taking Kiwi for a walk in a park between her exams.
- The original French name for this episode is Le lac.
- During this episode, all the characters return to their original attire from Seasons 1-3. The reason for this is unknown.
- In the English version of the episode, Yumi has a French language exam. In the original French dub, she has an English language exam.
- William manages to fight X.A.N.A.'s possession for the only time in this episode.
- This episode marks the only time in the original series where the Scyphozoa is destroyed. Mutiny later repeated this feat.
- The four towers activated by Aelita are in a more darker shade of green than usual.
- At one point while on the bus at the beginning of the episode, the heart on Sissi's shirt is missing.
- When Yumi is fighting with two Krabs, the red halo on the activated tower disappears.