In science class, Mrs. Hertz is teaching about nitrous oxide, more commonly known as laughing gas, and its effects. It can be dangerous if not neutralized by water quickly. Meanwhile, Yumi has been 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. Furious Sissi pours water on him, unaware of the cause of his maniacal laughing and tells him that if he doesn't know how to behave he has no place in her play. When Odd tells the others about his experience, they realize that X.A.N.A. must be behind it. They go to the science class and find the empty gas cans of nitrous oxide.
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 the play while Jeremie and Odd head to the Factory. Meanwhile on Lyoko, Aelita is being chased by two Bloks in the Mountain Sector, but she manages to hide inside an inactive tower. The X.A.N.A.-possessed gas goes after each member of the group. Odd enters the scanner and just as its doors are about to close, the gas goes inside the scanner, resulting in him being severely weakened once he arrives in the Mountain Sector. Jeremie is chased away from the lab by the gas and is forced to take refuge in the cold 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. A weakened Odd approaches the tower Aelita that has taken refuge in and successfuly manages to defeat the Bloks in front of it by using surprise attacks. Aelita, hearing Odd, leaves the tower and tells him to look for the activated tower before X.A.N.A. sends more monsters and the others laugh themselves to death.
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, only for the gas to return and attack them again. Back on Lyoko, Odd becomes weaker and weaker and Aelita realizes that they have to go through a labyrinth because the activated tower is on the other side. When they get lost in it and Odd is unable to move, he suddenly has a psychic vision and gives Aelita directions of where to find the tower. She manages to exit the labyrinth to the the tower and successfully 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 the play again. However, the gang reminds her that it's also the only 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 plays the guard this time, sparking a laugh at his appearance from the audience. When Yumi's father talks to his wife about what 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 sparks laughter from 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.
- This is the last episode in the series where Odd uses his Anticipation ability.
- When Odd is about to fight with two Bloks, the halo on the tower in which Aelita is disappears.
|Code Lyoko: Season 1|
|X.A.N.A. Awakens | Teddygozilla | Seeing Is Believing | Holiday in the Fog | Log Book | Big Bug | Cruel Dilemma | Image Problem | End of Take | Satellite | The Girl of the Dreams | Plagued | Swarming Attack | Just in Time | The Trap | Laughing Fit | Claustrophobia | Amnesia | Killer Music | Frontier | The Robots | Zero Gravity Zone | Routine | Rock Bottom? | Ghost Channel | Code: Earth | False Start|
|Season 1 · Season 2 · Season 3 · Season 4 · Evolution|