In science class, Mrs. Hertz is teaching the about nitrous oxide, more commonly known as laughing gas, and its effects. It can be dangerous if not neutralized by water quickly. Meanwhile, Yumi is in a very depressed mood for two days. The boys are unable to figure out what is going on with her and no one can seem to cheer her up, even Ulrich. Determined to find out what is troubling her, he shows up at her house the following night with a single rose. This gets her to admit to him how her parents have been fighting and her father has left home to stay with a friend from work.
During the rehearsal for the school play, X.A.N.A. possesses the laughing gas in the science room and uses it to attack Odd, who is doing the music. Sissi pours water on him, unaware of the cause of his maniacal laughing. When Odd tells the others about his experience, they realize that X.A.N.A. must be behind it.
Wanting to help bring Yumi's parents back together, Ulrich uses the school play of Romeo and Juliet as a way to do so. He reluctantly agrees to play the role of Romeo and Yumi decides to play a guard, even though she hates the costume. She says that she lied to her parents about it by saying the other wouldn't be coming so they'll be brought back together. Ulrich and Yumi stay behind in order to attend to the play while Odd heads to Lyoko. Just as he is about to go in, the X.A.N.A.-possessed gas goes after each member of the group. Odd gets to Lyoko's Mountain Sector, but the gas goes with him, weakening him severely. Jeremie is chased away from the lab by the gas and is forced to take refuge in the sewer water.
During the play, Ulrich and Yumi are both attacked with the laughing gas as it goes on to attack the crowd and audience. Aelita and a weakened Odd have to get to the tower before the others laugh themselves to death, but have trouble doing so.
At the school, the gas goes on to focus on only Ulrich and Yumi and her parents become frightened of their abnormal state. Sissi, due to her experience with Odd, pours water on Ulrich, who says he felt like he was being controlled. He tries to take the water from Sissi to help Yumi, who is still laughing uncontrollably. He manages to get it from her and pours it on Yumi stopping the problem. However, the gas returns and attacks them again. Odd and Aelita, while in Lyoko, continue to struggle to find the tower. However, Odd has a psychic vision and gives Aelita directions of where to find the tower. She manages to find the tower successfully and deactivates it, stopping the mayhem and saving Ulrich, Yumi, and Jeremie from choking to death on the laughing gas.
After a return to the past, Yumi declares she wants nothing to do with play again. However, the gang remind her that its also the way to reunite her parents. Jeremie then announces he has an idea on how to do the play without Yumi playing a guard. During the play, Yumi whispers Ulrich's lines to him from behind a stage prop of a bush while Herb this time plays the guard, sparking a laugh at his appearance from the audience. When Yumi's father talks to his wife about where Yumi's part in the play is, Jeremie pulls up the stage prop Yumi is hiding behind with along with Ulrich by pulling on the stage rope attached to it. This showing Yumi's appearance on the stage causing laughter by the audience, including Yumi's parents, making her smile.
- The original French name of this episode is Crise de rire.
- Yumi is revealed to have a stuffed Totoro (from the Studio Ghibli film) in her room in this episode.
- This episode was novelized in L'Aventure Continue.
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