In the dead of night, a manhole cover opens up. Team Lyoko climbs out and starts to leave. As Yumi runs off, Ulrich tells her, "Later". When he turns around, the other three are still there waiting for him. They urge him to make a move on Yumi, even offering to cover him in school so he can go out with her. Ulrich gives in and starts running after her.
When Yumi gets home, she finds William waiting for her. As William starts romancing her, Ulrich runs around the corner and sees the two. Horrified at the sight, Ulrich turns around and runs back to school without uttering a word. Seconds after Ulrich leaves, Yumi ducks out of William's attempt to kiss her and in an awkward moment, slams the front gate on him. William says, "It's Ulrich, isn't it?" She's silent for a moment before simply saying, "Goodnight." She leaves, going inside her house.
In Odd's dorm room, Jeremie and Aelita are listening to Odd's joke when Ulrich comes in. Odd asks how it went, but Ulrich stays silent and lies down on his bed. With the happy mood gone, Jeremie and Aelita leave. Odd asks again, but his question is met with a, "Leave me alone…"
The next day, the students are sitting in the bleachers next to the track as the principal tells them that they'll be trained in how to properly drive. Yumi says "hi" to Ulrich, but he stays silent, clearly ignoring her.
Principal Delmas then introduces them to the police officer who will be supervising their driving. He also introduces them to Jim, who salutes the police officer, stating,"Corporal Jim Morales, 7th Engineering Battalion, serial number 92121." The officer asks if he was in the army. Jim responds,"I'd rather not talk about it."
Yumi grabs a hold of Ulrich's arm, demanding an explanation for his sudden cold attitude towards her, but he tells her to leave him alone. Going to Odd for a reason, he explains to Yumi that Ulrich followed her home. Now, she knows why he's mad at her. The police officer tells the students that they'll be driving a training car around the track one by one.
Sissi is the first one to drive the car. Most of them have trouble driving the car, with problems from accelerating to the gear shift. When Ulrich gets in, he knocks over all the traffic cones. Odd makes a joke about his driving and Ulrich snaps at him. William tells him that it's not Odd's fault that he "drives like a dork", provoking him. Ulrich snaps, clenching his fists, and launches himself at William, getting into a fistfight with him. They are separated by Jim and taken to the principal's office.
In the principal's office, they are scolded and are threatened with suspension. Principal Delmas tells Jim to take them to the school library. Meanwhile, Jeremie detects an activated tower and the group, except Ulrich heads over to the the Factory.
Mr. Ishiyama comes home early from work and hears a large thump. Mrs. Ishiyama opens the front door and stares into the cold, lifeless eyes of a Krab. At the Factory, the kids see a Krab on the Factory floor, which begins to shoot at them.
Yumi gets a call on her cell phone. It's her little brother, Hiroki, who tells her that a "big monster" is outside the house. She tells the others and they let her leave.
Jeremie calls Ulrich, but his cell phone has been confiscated by Jim. Odd jumps to the floor below and distracts the Krab, while Aelita and Jeremie run to the elevator. He manages to get into the elevator along with the others. On a street of Boulogne-Billancourt, a police officer tells Yumi that a dangerous animal is on the loose and that for her own safety, she cannot pass through.
Odd and Aelita find two of the three Scanners destroyed. Aelita goes into the last Scanner first, appearing in the Mountain sector, along with the Overboard. When the Scanner opens up, a Krab emerges.
In the library, William, Ulrich, and Jim hear laser fire and screaming. Jim looks out the window and sees kids running. He tells the two detainees to stay put while he finds out what's going on. William looks out the window and sees an orange killing machine. He wonders what it is. Ulrich ominously responds,"A Krab".
At the Factory, the last scanner is destroyed by the Krab. Odd climbs up the ladder, into the Lab. On Lyoko, Aelita takes off on the Overboard. Meanwhile, the police officer hears radio chatter that the creature has been spotted in the vicinity of Kadic and drives off, leaving no one to stop Yumi from getting to her house.
Aelita is still getting the hang of the Overboard. Ulrich calls Jeremie, asking him what's going on. Jeremie tells him that three Krabs have come out of Lyoko. One is at Kadic, another at Yumi's house, and the third at the Factory. Jim is knocked out by a laser from the Krab as Ulrich and William run outside. William distracts the Krab while Ulrich jumps down from a tree onto the Krab and jams a metal stick into its Eye of X.A.N.A., destroying it. Meanwhile, Aelita uses Creativity to generate a railing and slides atop it like a skateboarder.
Odd is climbing up the elevator shaft when he hears a bang. He looks down and realizes that the Krab has destroyed the elevator, but keeps on climbing up. At Kadic, Ulrich tells William to stay away from his group and that he's lucky he saved him. William tells him that in "the race for Yumi", he's not exactly in first place. This clearly confuses Ulrich.
Ulrich then starts running to Yumi's house to deal with the Krab there. Yumi enters her house, finding it unnervingly quiet. She goes upstairs and is almost attacked by her father, who is dressed up in samurai attire. At the Factory, Odd notices that the Krab is climbing up the elevator shaft. As this is happening, a squadron of Hornets comes up behind Aelita and starts firing at her. She flies around a mountain and accelerates her Overboard to ram a Hornet.
The disoriented Hornet fires out of control and destroys the Hornet in front of it. As Aelita flies around another mountain, six more Hornets enter the fray.
They separate their flight formation and split into groups of three, each one rounding the mountain in opposite ways. Aelita flies out of the way, causing two Hornets to collide, and rockets up the mountain. She then noses the Overboard downwards and flies past the group of five Hornets. As she flies onto another path, she notices a hole ahead. She back-loops into the hole and flies down into the fog layer.
The Hornets attempt to mimic her moves, but two of the Hornets run into the rim of the hole and explode. The remaining three continue to pursue her. Meanwhile, at Yumi's house, Yumi is shot in the leg by the Krab. Mr. Ishiyama attacks the Krab, but he is knocked aside, and drops his katana. At the Factory, the Krab follows Odd into the Assembly Room, while at Yumi's house, the Krab is about to kill Yumi with a laser to the head, but the laser is deflected by a katana being wielded by Ulrich. With the words "Make my day, shrimp.", Ulrich begins to attack the Krab.
On Lyoko, Aelita flies into a tunnel. Two Hornets follow her in, while the third enters the tunnel from the other side. One of the Hornets runs into a rock and is destroyed, while the other careens into the side of the tunnel and struggles to get up. Moments later, the Hornet gets up and continues to pursue her. Aelita reaches a chasm with rocks below and two tunnels leading into it. With her elf ears, she detects the flapping of the approaching Hornets, which are coming in from both sides. She tells Jeremie to devirtualize the Overboard and lands on the rocks below, as the last two Hornets run into each other. Jeremie brings up the Overwing and she flies out of the tunnel.
As Odd runs from the Krab, William drives in with the training car. He drives Odd out of the immediate line of fire and asks what they should do now. Odd tells him that he has a plan. On Lyoko, Aelita emerges out the tunnel and the activated tower is in sight, but a Tarantula and the Scyphozoa are waiting for her. The Tarantula shoots down the Overwing as Aelita tries to escape. She gets up and starts running back into the tunnel, with the monsters in pursuit. At the Factory, Odd ties a rope around the leg of the Krab. That same rope is tied around the passenger seat of the car.
William floors the accelerator and drives the car toward the elevator shaft, dragging the Krab with it. William dives out of the car as it slides into the elevator shaft and falls in. It seems the Krab is about to be dragged down the shaft too, but one of its legs grabs hold of a support beam. Moments later, it loses its grip and falls to its demise.
Meanwhile, on the street, Ulrich slices off one of the Krab's legs. Hiroki, dazzled by the action, runs into the middle of the fray. Ulrich tells him to stay back, but gets shot by the Krab. On Lyoko, the Scyphozoa catches up to Aelita and wraps its tentacles around her. However, she seemingly disappears, revealing her as a decoy.
The real Aelita runs out of the tunnel and dodges laser fire to enter the tower. At the Ishiyama house, the Krab has Ulrich pinned down with one leg and has another raised in preparation to decapitate him, but Aelita manages to deactivate the tower.
The Krab's sharp leg stops, centimeters from Ulrich's head. Yumi exhales a sigh of relief, while Jeremie activates a Return to the Past, repairing the Scanners and elevator and wiping everyone's memory of the incident.
It takes everyone back to the driving test. Ulrich steps out of the car and William compliments him on his driving. William asks him to join his go-kart club. Ulrich agrees to join, much to Yumi's surprise.
- The original French name for this episode is Mauvaise Conduite.
- Despite being numbered as episode 39, it was aired before Temptation (episode 38) and after episodes 41-47. The reason for this is unknown.
- This is the only episode where Aelita completes a mission all on her own without her friends going to Lyoko.