'Quest for the Mushroom' is episode 4 and consists of 2 parts. In this episode, Luigi becomes injured, and Mario must find the mushroom to help him!
- The Lizard of Doom (debut)
- The Ehh Guy (debut)
- The You! Guy(debut)
- Bullet Bill
While Mario and Luigi are making Miis of themselves on the Wii, Mario says to Luigi that he is bored and suggests going to Death Forest for fun. Mario and Luigi then ride a bike to Death Forest and go separate ways to different things in the forest.
Mario gets a suntan, while Luigi climbs a tree. While climbing the tree, Luigi is hit by a bullet bill and struck off the tree. The bullet bill rides away trying to look like he never harmed Luigi. Mario then brings an unconscious Luigi to his house, only to find Sylvester the Cat from Looney Tunes who says he knows of a cure that can help Luigi get revived; a medicine mushroom.
Mario is given a map telling him to go to Death Forest and another place. Mario goes to these places, defeats the demons at these places, and defeats the lizard guarding the mushroom. In the end, Mario gives the mushroom to Luigi and saves his life. They also become friends with Bullet Bill. But, however Bullet Bill accidentally strikes Luigi again, then he looks at the camera and says "Looks like, we'll need another mushroom". Then, the episode ends
- In this episode,you can see Tyler's face,so Mario had to be voiced by Ricky.
- The You! Guy And The Ehh Guy first appeard in this episode.