The episode begins as Odd, Ulrich, and Jeremie are walking down the hallway of the dormitory building. Jeremie is trying to count the number of girls Odd has dated. Odd reveals to them that he has dated all the girls in 8th grade, except for Heidi Klinger. As they walk past Sissi and her group, Sissi asks Ulrich to see a movie called "Adventure in the Tropics" with her. Immediately, Ulrich says he's already seen it and leaves. Sissi is disappointed and Herb says he'll go with her. Sissi sarcastically agrees, wiping that giddy look off Herb's acne-ridden face. Meanwhile, in Lyoko's Desert Sector, X.A.N.A. activates a tower.
In the shower room, Odd asks if Ulrich really saw the movie and Ulrich responds that he'd rather study for a math test than go on a date with Sissi. Herb, overhearing Ulrich, says he'll knock that "smile" off his face. Ulrich, still calm, tells him to go ahead. Herb then kicks around aimlessly in the air and accidentally karate chops the sink. In pain, Herb retreats from an embarrassing defeat. Jeremie leaves, saying he promised Aelita his croissants for her to eat since she loved eating them with her hot chocolate. In the showers, Odd tells Ulrich that he got a date with Heidi Klinger. Outside, one of X.A.N.A.'s ghosts emerge.
In front of the cafeteria, Jeremie walks up to and kisses Heidi Klinger. She asks if he's seen Odd since he promised to have breakfast with her, but he grabs her and kisses her rather passionately as Odd and Ulrich arrive and see the whole event. He then runs off. Odd is shocked that Jeremie kissed his date in plain view. Meanwhile, in front of the vending machine, Yumi comes up to Mathias Burrel and kisses him while William comes across the scene and watches in obvious shock. As Aelita and Jeremie walk, they pass by Heidi, who asks Jeremie why he kissed her in front of Odd. After hearing this, Aelita walks away from Jeremie who denies such an event ever happening.
In the cafeteria, Jeremie comes to the table where his friends are, but tensions are high. Odd, angry at him for kissing his date, and Aelita, mad at him for kissing another girl, leave immediately. Ulrich gets out of his seat and takes Jeremie's croissant as he leaves, saying he won't be needing the croissant anymore since he intended to give it to Aelita.
As Aelita, Odd, and Ulrich leave the cafeteria, William comes up to Ulrich and tells him that Yumi's going out with Mathias Burrel. Shocked at the news, Ulrich puts down his croissant as Yumi comes up to them. William and Ulrich start grilling her with questions on Mathias. Yumi says, "He's a nice guy" and asks why. William bluntly asks, "How does he kiss, huh?" Yumi, stunned, asks why they were asking her such a question when Ulrich tells her not to deny it as William saw her. Her two lovers walk away, leaving Yumi confused as to what's going on. She asks Jeremie what the rumor is about her and Mathias. Jeremie then tells her about the rumor that he kissed Heidi Klinger.
Meanwhile, in the bathroom, a polymorphic clone changes from Yumi to Ulrich. Outside, Herb is trying to convince Sissi that Ulrich doesn't want to go out with her, saying he said so himself. Unfortunately for Herb, Ulrich walks up to Sissi and kisses her. Jeremie and Yumi then watch the entire event from the distance, leaving Yumi stunned.
In history class, things boil over. Ulrich asks Jeremie why he kissed Heidi, saying it wasn't a very nice thing to do to Odd. Jeremie denies it and when Ulrich states that he saw it, he accuses Ulrich of kissing Sissi in order to get even with Yumi. Ulrich, clearly surprised at the accusation, claims it never happened and Jeremie uses the same excuse they did for him with Heidi, telling him, "I saw you". The teacher goes on with the lesson as Ulrich remains shocked. The teacher, noticing Ulrich's distraction, picks him for a question and asks, "When did Gallume die?" Ulrich comes up with, "At the end of his life?"
Outside, Jim, who is on patrol, sees Ulrich who is supposed to be in class, and chases him. Ulrich runs around the corner and Jim follows him. However, he sees Yolanda Perraudin, the school nurse, passing by. She walks up to him and kisses him, much to his pleasant surprise. After class, Jeremie sees as Aelita kiss Nicholas, who is dumbstruck. Later, Aelita comes up to Jeremie in the Dormitory after being told by him she wanted to speak with her. He accuses her of making a fool of him in public and of lying to him. Jeremie walks into his room and slams the door. Aelita then runs away. In his room, Jeremie detects an activated tower. He calls Yumi, Odd, and Ulrich; who, meanwhile, is trying to convince Yumi he never kissed Sissi, to tell them about it. He also tries to call Aelita, but she isn't picking up.
In the park, Jeremie explains that all the kissing is because of a polymorphic clone, which can change its identity at will. The purpose: to divide and conquer. This causing Jeremie to realize Odd had been right when saying that "jealousy makes people crazy". Now that they know the truth, all they need is Aelita. They don't know where she is, but Jeremie has an idea.
At the Hermitage, Aelita is moping in the torn-up remains of her old childhood room. Jeremie comes in and apologizes to her, explaining that it was a polymorph that was responsible for all the drama. He kisses her passionately and they run out of the house, hand in hand.
Outside, they encounter another Jeremie. They accuse each other of being the polymorph. Ulrich, Odd, and Yumi catch up, so they don't know which is the real Jeremie. Aelita begins contemplating and realizes that the real Jeremie wouldn't have kissed her and runs over to the other one. The polymorph then grabs Aelita and tries to attack her, but is temporarily stopped by Odd. Odd stays behind to fight the polymorph as the others take the Hermitage's secret path to the Factory. The fight quickly degenerates into a losing one for Odd, once the polymorph transforms into Jim.
At the Factory, Jeremie virtualizes Aelita, Ulrich, and Yumi into the Desert Sector. The activated tower is on the next plateau connected only by a narrow strip of land, which forms a bridge. They begin to run towards it.
In the Hermitage, Odd is getting knocked around. He says, "Hey, X.A.N.A., if you're going to keep changing appearances like that, do me a favor and turn into a wimp next time." The polymorph obliges his opponent's wish by transforming into Odd. Odd responds, "Oh, no. Not him. I know his kind. Small, but tough as nails." The polymorph Odd beats him up even more now that they're evenly matched.
On Lyoko, Aelita continues toward the tower, while Yumi and Ulrich fight an approaching swarm of Hornets. X.A.N.A sends a Tarantula to fight them. Immediately after Aelita runs across the bridge to the next plateau, X.A.N.A deletes it, separating her from the other two. Meanwhile, the polymorph runs across the bridge, into the Factory.
Yumi fights the Tarantula while Ulrich fights the Hornets. However, they have trouble and decide to swap enemies. Jeremie brings up the Overwing and Yumi hops aboard, with the Hornets in pursuit. She throws her tessen fan in a circle around her, destroying the Hornets. Ulrich uses Super Sprint to get behind the Tarantula, but loses his advantage when two more Hornets show up. Yumi swings by to attack the Tarantula, but she is shot down and devirtualized by laser fire. Ulrich destroys one of the Hornets, but he still has two more monsters to go. Yumi comes out of a scanner and waits at the door as the elevator is in use.
On Lyoko, Aelita is trapped as her escape route has been deleted by X.A.N.A. The Scyphozoa arrives and begins to take her memories. In the elevator, the polymorph Odd transforms into Yumi to coincide with her return from Lyoko. Jeremie brings up the Overbike so Ulrich can cross the gap and get to Aelita. The polymorph Yumi comes into the Lab and tosses Jeremie aside.
Outside, the real Odd has come to get revenge. The polymorph Yumi devirtualizes the Overbike before Ulrich can get on, forcing him to use Super Sprint to jump across the gap. Meanwhile, Odd arrives in the Lab to see two Yumi's. One of them calls Odd a pea-brain. Odd turns and faces that Yumi, but when it seems they're about to fight, they turn and attack the other Yumi. They knock out the polymorph temporarily and get Jeremie. On Lyoko, Ulrich throws his katana like a boomerang, severing the Scyphozoa's tentacles and returning Aelita's memories. As Aelita enters the tower, the polymorph reawakens, tosses Yumi and Odd aside and starts for Jeremie. As Jeremie gets electrocuted by the polymorph, Aelita deactivates the tower. The polymorph turns back into a spectre, which rises up and disappears into the lab's ceiling.
In the infirmary, Jim is offering Yolande Perraudin a flower. She is slightly flattered, but she explains that she is engaged to be married. Jim then asks why she kissed him if she is engaged. Yolande, confused, asks if she really kissed him. He responds by nodding up and down like a fool causing her to think he must be ill, so she instructs him to get on the bed so she can take his temperature, much to his disappointment.
Outside, Yumi asks Odd how he knew she was the real Yumi. Odd tells her that only the real Yumi would call him a pea brain not X.A.N.A. since he has too much "respect" for him. Jeremie then claims he just doesn't know him like his friends do. Aelita then asks Jeremie if he could come with her for a moment. Jeremie, though a little curious, is happy to oblige and is met with a real kiss from Aelita, much to his shock. He remains in a frozen pose long after Aelita walks away (smugly saying "I knew it") and his friends laugh in amusement, Odd even commenting that X.A.N.A. should hold back from attacking for a while, simply because of how long Jeremie would be frozen in his pose.
- The original French name for this episode is Mon meilleur ennemi.
- Chronologically, this episode takes place after both Temptation and A Bad Turn. However, it aired weeks before either.
- At the beginning of the episode, Odd mentions that Heidi is the only girl in the 8th grade with whom he has not gone out. This is technically not true; there still remain Aelita and Sissi.
- In the Indonesian dub, and possibly even some other countries' dubs, all of the kissing scenes were cut out due to sexual references; all of this censoring leaves this episode's entire plot to be obscure.