In class, Mrs. Hertz is teaching the class about nitrous oxide, more commonly known as Laughing Gas, and its effects. It can be dangerous if not neutralized by water quickly. Meanwhile, Yumi's parents are having trouble with their relationship, which Yumi isn't too happy about. No one can seem to cheer her up, even Ulrich. Determined to somehow cheer her up, he shows up at her house the following night with a rose, eventually managing to get her to talk about her parents trouble.
During the rehersal for the school play, X.A.N.A. possess 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 her find a way to bring her parents back together, Ulrich uses the school play of Romeo and Juliet as a way to do so. He reluctantly agrees into playing Romeo and Yumi decides into playing a guard and says that she lied to her parents about it by saying the other wouldn't be coming to the play to see her so they'll be brought back together. Ulrich and Yumi stay behind because of 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, but the gas goes with him, weakening him severely. Jeremie is chased away from the factory 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 Jeremy from choking to death on the Laughing Gas.
After a return to the past, Yumi declares she wants nothing to do with play but the gang remind her that its also the way to get her parents reunited. Jeremy then announces he has an idea without Yumi playing a guard. During the play, Yumi whispers Ulrich's lines to him from behind a stage prop of a bush while Herve 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 is, Jeremy 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.
This episode contains examples of:
- All Part of the Show: Jeremie comes up with a different idea to keep Yumi's parents from splitting up; still invite them to the Romeo and Juliet play as per Ulrich's initial plan, but make an intentional "accidental" scene change where Yumi and Romeo!Ulrich are hiding behind a bush, which prompts the audience to laugh at the mistake. Sissi still gets mad at the laughter.
- Bad "Bad Acting": Although Ulrich's voice actress was a former Broadway Star it took a lot of guts to make Ulrich act that badly...twice.
- Broken Angel: Laughing Gas in the scanner makes Odd disoriented and weak throughout his mission with Aelita. He fares decently well for a good while and wisely starts outwitting the monsters instead of fighting them head-on, he can't keep it up for very long and breaks down in the labyrinth.
- Die Laughing: What XANA intends to do to the heroes. Mrs. Hertz even drops the trope title by name.
- Helpless with Laughter: The victims of XANA's plan fall to the ground, unable to control their laughter, as they nearly suffocate to death.
- Irony: In an episode that uses laughter as a weapon (X.A.N.A. using Laughing Gas), it otherwise has the team jokester, Odd Della Robbia, be at arguably the most unfunny he's ever been during his time on Lyoko due to the gas sapping most of his strength.
- Just in Time: Aelita manages to trigger the return jump seconds before Jeremie succumbs to the gas. Curiously, Jeremie is shown in extreme pain, but somehow still says "Return to the past now!" in an assertive voice.
- The Maze: One of the things between Aelita and Odd and the activated tower.
- Missing Mission Control: The second time in the series for Jeremie goes away from the comms, though Justified in that he had to run away from the XANA-laughing-gas, and didn't have time to grab a phone, and so was unable to help Odd or Aelita.
- Morton's Fork: Jeremie eventually finds himself in this predicament; he's in freezing-cold water that he can't climb out of without succumbing to the gas, and starts pondering whether dying via drowning or laughing is worse.
- Mrs. Exposition: Mrs. Hertz explains Nitrous Oxide to the students in the science lesson, including explaining the Weaksauce Weakness to cure the laughing effects; Water.
- O.O.C. Is Serious Business:
- Odd and Jeremie have never seen Mrs. Hertz laugh (in Jeremie's case, he hasn't seen her laugh so much, but to Odd, he hasn't seen her laugh at all.) Luckily, after a drink of water, the serious teacher side returns and she orders a surprise quiz.
- Ulrich forcefully asking Sissi for water gets him a mean look, but asks again in a more assertive tone, and Sissi's attitude changes completely and gives the water to Ulrich.
- Odd himself is noticeably less animated upon his botched virtualization to Lyoko. His sense of humor remains somewhat, but the vulnerability in his tone makes it clear that this isn't a side of him we've seen before. He even comes close to giving up after some time in the labyrinth until he has a vision and points Aelita the rest of the way.
- School Play: 1 of 2. The play: Romeo and Juliet.
- Title Drop: Jeremie says "A good laughing fit is good for you from time to time" at the end of the episode.
- Weaksauce Weakness: The laughing gas is nullified by ordinary water. This proves problematic near the end of the episode where Yumi and Ulrich can't stop laughing and nobody has water to relieve them, while Jeremie is forced to flee on foot due to being swimming in the sewer water for too long.
