The episode begins in Carthage, with Aelita collecting data at the interface in the Celestial Dome while Yumi, Ulrich, and Odd battle several swarms of Mantas. Aelita finishes her work and uploads the data to Jeremie before the group escapes the remaining Mantas on their vehicles and Jeremie manually devirtualizes them, bringing them back to Earth.
The group convenes in the Lab, where Jeremie is confident this new batch of data will allow them to decrypt Franz Hopper's diary. Aelita mentions that they will need to activate a tower in order to run the decryption program, which could be dangerous, but Jeremie insists there is no other option. Jeremie stays in the Lab to prepare for the next mission while the others take a break for lunch.
In the school cafeteria, Odd, Aelita, and Ulrich sit down together and discuss their grade point averages until Ulrich makes a faux pas by mentioning Aelita's status as AI. Aelita excuses him, saying she is comfortable with the fact that she is not human, but confesses to finding Earth familiar, as if she had lived here once before.
After lunch, Odd receives an anonymous text message from a girl who wants to meet him in the woods, and leaves Ulrich and Aelita to meet her. At the Factory, Jeremie is alerted to an activated tower and calls the others, asking them to come back. Ulrich and Aelita decide to leave without Odd, and return to the Factory with Yumi. In the woods, the mysterious text is revealed to be a trap, and Odd is attacked by a spectre.
Jeremie calls Odd's phone while Yumi, Ulrich, and Aelita go down to the Scanner Room, but no one picks up. Giving up on Odd, Jeremie sends the other three to the Mountain Sector. Just then, Odd arrives in the Scanner Room, acting uncharacteristically surly, and is virtualized as well. During Odd's virtualization, Jeremie detects a bug, but before he can do anything about it, he is distracted when the activated tower shuts itself down for unknown reasons. With the threat gone, the group decides to go along with the original plan of activating their own tower and running the decryption program. Ulrich, Aelita, and Yumi begin to notice Odd's strange behavior as Jeremie virtualizes their vehicles and directs them to the designated tower.
They arrive at the tower, and Aelita enters while Ulrich, Odd, and Yumi wait outside. Aelita and Jeremie work together to activate the tower and start the decryption program. Three Tarantulas appear suddenly, but before the battle starts, Odd reveals himself to be a Polymorphic Clone and turns on the others. Yumi is devirtualized, but Ulrich manages to escape on the Overbike. Meanwhile, at the Hermitage, it is revealed that the real Odd has been knocked out and dumped inside a water drain that is slowly being filled with water from a garden hose. Odd wakes up and desperately tries to escape, but cannot climb high enough.
On Lyoko, Ulrich is pursued by the Odd clone on the Overboard while the Tarantulas guard Aelita's tower. Back on Earth, Yumi leaves the Factory to find the real Odd while Jeremie tries to abort the mission. However, X.A.N.A. begins attempting to take control of Aelita's tower, and Aelita discovers the decryption program cannot be stopped; if X.A.N.A. gains control of the tower while the program is running, he will be able to access sensitive material stored in the Supercomputer and destroy all the Franz Hopper's and Jeremie's work.
Ulrich returns to the tower and defeats two of the Tarantulas before attempting to severe the tower's cables, which would prevent X.A.N.A. from gaining control of it. However, he is stopped by the clone, which transforms from Odd into a second Ulrich. Meanwhile, at the Hermitage, the water has risen high enough for Odd to reach the grate at the top, but it is bolted to the ground, and he is not strong enough to pull it open. Yumi, however, is in the area, and decides to check the abandoned house.
On Lyoko, Ulrich furiously battles his clone while Aelita tries in vein to stop the program and X.A.N.A. comes severely close to taking control of the tower. Ulrich manages to defeat his clone, and races back to the tower to cut the cables, but he is devirtualized by the last Tarantula before he can do so.
At the Hermitage, Yumi shuts off the garden hose and finds Odd almost completely submerged. She tries to pry to metal grate off the drain while Odd struggles to breathe. On Lyoko, X.A.N.A. nearly takes control of the tower before being overpowered at the last moment by Franz Hopper, who turns the tower white (like in the previous episode) an allows the decryption program to run to completion. Meanwhile, Yumi finally manages to rip the grate off the drain and rescue Odd. Franz Hopper releases the tower from his control, and Aelita returns to Earth.
Some time later, the Lyoko Warriors gather in Jeremie's dorm room, where he recounts for them the things he has learned from Franz Hopper's diary so far. He confirm their suspicions that Hopper once lived on Earth, and that he created X.A.N.A. while in the process of building Lyoko. He then reveals to them that Franz Hopper had a daughter who was virtualized with him onto Lyoko. Aelita struggles to comprehend this at first, but eventually realizes that she is not an artificial intelligence, but is in fact Franz Hopper's daughter.
- The original French name for this episode is Révélation.
- This episode premiered as a double bill along with The Key.
- This is the first episode where a green tower is depicted on the title card.