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 to 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 a return in time. The others ask why, seeing as X.A.N.A. hasn't attacked Earth yet. Not only that, return trips are known to strengthen X.A.N.A.. Jeremie doesn't bother answering and activates the program anyway.
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 realize 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 then detects an activated tower and Jeremie declares, "You see?! I told you X.A.N.A. never takes a break!"
At the cafeteria, Yumi gets a call from Jeremie, who tells her that X.A.N.A. has launched another 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 X.A.N.A.. The three duck below their table. Odd and Ulrich get up and start throwing plates at Rosa 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 Kankrelats are blocking the path. As they hide behind a boulder, Jeremie brings up the Overwing. At the cafeteria, Rosa has picked up a food cart and is about to hurl it at the boys. On Lyoko, Yumi uses her Telekinesis to bowl over several Kankrelats and uses the Overwing to drop off Aelita at the activated tower. Aelita deactivates it and Rosa 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 return in time again. At the vending machine, the gang wonders why Jeremie hasn't come yet. They go to his room, only to 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 returns to the past. 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 the return trips. 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 X.A.N.A. doesn't attack Jeremie.
Outside the Administration Building, Jeremie is put into an ambulance. One of X.A.N.A.'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, Ulrich, Aelita, and Odd 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.
Meanwhile on Lyoko, a group of Tarantulas confront the Lyoko Warriors. 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 out of five 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 her 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 another return trip. 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.
- The original French name for this episode is Tentation.
- This episode, despite being numbered as 38, and chronologically taking place before episodes 39-49, it airs considerably later than all of them. It airs in conjunction with episode 50, Contact.
- When Odd and Ulrich are being attacked in the cafeteria by a possessed Rosa, Ulrich makes a reference to the cartoon Looney Tunes, saying "Now I know what Tweety feels like against Sylvester".
- This episode was released in book form, but only in French.
- Before the first return in time, Odd asked Jeremie if he was sure, but Ulrich's audio card made it seem like he was talking.
- When X.A.N.A. materializes the Tarantulas in the Ice Sector, it is shown that he sent two of them. However, in the next shot, five Tarantulas suddenly appear.
- When Ulrich is asked to do a return to the past, he states that he doesn't know how to activate it despite the fact that he successfully launched a return trip a few episodes earlier in The Chips Are Down.