Laughing Fit.In class, Susan teaches the class about nitrous oxide, more commonly known as laughing gas, and its effects. Meanwhile, Yumi's parents are having trouble with their relationship, which Yumi is not too happy about. No one seems to be able to cheer her up, even Ulrich. He eventually manages to do so, however, and helps her find a way to bring her parents back together. During a school production of Romeo and Juliet, 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. The play turns out to be the perfect way to get Yumi's parents back together, so Ulrich, who is forced into playing Romeo, and Yumi, who plays a guard, stay behind while Odd heads to Lyoko. Just as Odd 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. Jeremy is chased away from the factory by the gas and is forced to take refuge in the sewer water. Finally, Ulrich and Yumi are attacked during the play. Aelita and a weakened Odd have to get to the tower before the others laugh themselves to death.