The episode begins as Odd and Aelita weave through the trees of the Forest Sector on the Overboard. They are being pursued by a pair of Hornets. One of the Hornets runs into a tree while the other is destroyed by a passing tessen fan. It returns to Yumi, who is flying alongside them. On the ground, Ulrich is being followed by two Hornets. He hits the brakes on the Overbike and slashes them with his katana. A Hornet swoops in, destroying Odd's Overboard. Odd stays behind to fight off an incoming squadron of Hornets while Aelita runs toward the tower. Aelita hops across tree trunks to get to the tower, but the Scyphozoa is blocking her. Yumi swoops in on the Overwing and slices off its tentacles. Aelita deactivates the tower and Jeremie tells them that he is starting up the time reversion. The others ask why, seeing as XANA hasn't attacked Earth yet. Not only that, time reversions are known to strengthen XANA. Jeremie doesn't bother answering and activates the time reversion. At school, Aelita, Odd, Ulrich, and Yumi stand by the vending machine. They talk about Jeremie's increasing arrogance. Jeremie walks over to the vending machine and they ask where he was. Jeremie cockily responds,"In my room, why? Is there a law against it?" Then, the bell rings and everyone goes to class.
In Mrs. Meyer's class, Jeremie daydreams of walking in a white hallway that is being covered by darkness. Disoriented, Jeremie asks Mrs. Meyer if he can go to the Infirmary. At lunch, Odd, Ulrich, Yumi, and Aelita find out that Jeremie missed History and Gym class. Aelita leaves the Cafeteria to go look for him. She walks into Jeremie's room, finding him typing away at his computer. Jeremie berates her, telling her that he's been working on Aelita's antivirus, which is very important to him and to her. The Superscan detects an activated tower and Jereme declares, "You see?! I told you XANA never takes a break!" At the cafeteria, Yumi gets a call from Jeremie, who tells her that XANA has launched an attack. As they get up to leave, one of the cafeteria workers, Rosa Petitjean, tells them to sit down and starts throwing plates at them. She has been possessed by XANA. The three duck below their table. Odd and Ulrich get up and start throwing plates at Rosa Petitjean to cover Yumi's escape. Yumi gets to the Factory and is virtualized into Lyoko's Mountain sector, along with Aelita. They see the activated tower, but eighteen Roachsters are blocking the path. As they hide behind a boulder, Jeremie brings up the Overwing. At the cafeteria, Rosa Petitjean has picked up a food cart and is about to hurl it at the boys. On Lyoko, Yumi uses Telekinesis to bowl over several Roachsters and uses the Overwing to drop off Aelita at the activated tower. Aelita deactivates the tower and Rosa Petitjean drops the food cart and collapses. Odd tells Jeremie on the phone, "Jeremie, I think this calls for a return to the past. Don't you?" Jeremie then activates the time reversion again. At the vending machine, the gang wonders why Jeremie hasn't come yet. They go to his room and see him lying unconscious on the floor. At the Infirmary, Jeremie appears to be in a coma, but is dreaming. In his mind, Jeremie is walking along a white hall as darkness from behind him races forward. He tries to outrun it, but it overtakes him, leaving only a white light in the distance. Aelita and Odd go to the Factory while Yumi and Ulrich go to Jeremie's room to look for clues. Ulrich comically says that he hasn't found anything except the fact that Jeremie still wears underwear with kangaroos printed on them. On Jeremie's computer, Yumi tries to access a file called "My Journal", but it's restricted. At the Factory, Aelita can't find anything wrong with the time reversion program. She then gets a call from Ulrich, telling her to come back to Jeremie's room.
Aelita manages to bypass the restrictions on Jeremie's "My Journal" file, revealing six video clips. She clicks on the first one, bringing up a video log of Jeremie. In it, Jeremie explains that he decoded another part of Franz Hopper's diary, finding a way to exponentially increase his intelligence, using the unfathomable power of time reversions. He shows an erronal headset, which he uses for this. Then, Aelita accesses yesterday's log entry. In it, Jeremie says that his mental and physical health is deteriorating every time he uses the erronal headset, but he is unwilling to stop, so close to success. Odd, Ulrich, and Aelita go to the Factory while Yumi goes to make sure that XANA doesn't attack Jeremie. Outside the Administration Building, Jeremie is put into an ambulance. One of XANA's ghosts enters the ambulance and starts it up, before the driver can get in. Yumi gets there, just in time to see the renegade ambulance drive off. She starts pursuing the ambulance on a bike. At the Factory, Odd, Ulrich, and Aelita enter Lyoko's Ice Sector. They start running towards the activated tower. Yumi almost reaches the ambulance's door, but the ambulance speeds ahead, leaving Yumi behind. She rides her bike down a stairway and catches up with the ambulance. She opens the door and gets in, calling out to an unconscious Jeremie. In Jeremie's mind, Yumi's voice brings Jeremie out of the dark hallway and he wakes up. They plan on stopping the ambulance before they get off. Unfortunately for them, the ambulance is speeding toward the river.
In Lyoko, a group of Tarantulas confront Team Lyoko. Ulrich comes up with a plan. They fire around the monsters, causing cracks to appear in the ice. The cracks widen into fissures, with water between them. The ice breaks apart and most of the Tarantulas drown, leaving two left. In the ambulance, Jeremie smashes the window with an oxygen tank and Yumi gets into the driver's seat. Unfortunately, nothing in there is working. The ambulance crashes through the security railing and plunges into the river. Ulrich is devirtualized by a laser bolt and heads up to the Lab. Odd causes a Tarantula to slip off its ice piece and drown, but is devirtualized by the remaining Tarantula. In the lab, Ulrich responds to a desperate call by Yumi, who tells him that the ambulance is sinking. Aelita uses Creativity to make an ice arch that spans the icy water. She slides across it and enters the activated tower, deactivating it. In the ambulance, water is seeping in. The only thing that will save them now is a time reversion. Following Jeremie's instructions, Ulrich initiates a return to the past.
The group goes to Jeremie's room to see him smashing his headset. He tells them that most of the calculations he made were wrong. Jeremie admits that Franz Hopper's discoveries are out of his league. He'd better be careful the next time he tries out one of Hopper's experiments.
This episode contains examples of:
- Adults Are Useless: Principal Delmas and Yolanda saw the ambulance leave on its own with Jeremie plus Yumi pursuing it on her bike. They take no action to inform the police nor the Ishiyamas and just disappear for the remainder of the episode.
- All for Nothing: The neuronal headset failed to increased Jérémie's intelligence, as the calculations he made ended up being mostly incorrect, meaning that he put his health at risk for no gain towards finding an anti-virus. Lampshaded by Odd at the end.
- Apocalyptic Log: Jeremie stars maintaining a private video log inspired by Franz Hopper to detail his time when researching for the anti-virus, which essentially turns into a review of the neuronal headset he built.
- Continuity Snarl: Ulrich doesn't know how to activate the "Return to the Past" function, despite there being an entire episode based around Ulrich misusing it to make Yumi feel better about possibly leaving the country, which was only three episodes prior to this one airing. The episode even gave a Handwave of Jérémie compiling a manual just in case he's M.I.A.
- Cybernetics Eat Your Soul: Jérémie uses a neuronal headset based on Franz Hopper's notes to increase his intelligence, to help find an antivirus to cure Aelita and to fight XANA. However, the headset ends up worsening his physical and mental health.
- Dead Man Writing: Franz Hoppers' notes get referenced again, and Jérémie uses a neuronal headset to increase his brainpower.
- Foreshadowing: Jérémie acts really strangely for the first few minutes of the episode, acting irritable, unsociable, and even almost losing Aelita. This snaps him back to reality a bit, and it's implied Jérémie was going to tell her why he was acting so weird, had it not for an interruption to the conversation.
- Hero Stole My Bike: Yumi takes a bike on campus to chase after the XANA-possessed ambulance with Jérémie inside.
- The Man in Front of the Man: XANA is actually not the one who convinces Jérémie to use the headset, rather Franz Hopper. But XANA does exploit Jérémie while he's weak, and attempts to drown Yumi and Jérémie.
- Mission Control Is Off Its Meds: Essentially what happens to Jérémie by following Franz Hoppers' notes that he doesn't understand, and the price to pay was his mental state.
- Trapped in a Sinking Car: Ambulance, but same difference. Jérémie and Yumi.
- Vehicular Sabotage: XANA possesses the ambulance Jérémie is put in, and as Yumi finds out when trying to stop it from cashing into the river, disabled the brakes.
- What Happened to the Mouse?: The adults straight up vanish after the ambulance drives off with Jeremie and no action is taken against Yumi for getting involved in what is basically a high-speed chase through the Parisian streets.
- White Void Room: Jérémie is shown to be walking down a white hallway in his mind after using the neuronal headset. However, the hallway is shown being swallowed up by darkness, representing his deteriorating mental and physical health.
