In the game, the four Mario Bros. of past and present manage to stumble upon Princess Shroob's speech in the Koopaseum, with the Shrooboid Brat being called in to defend the princess.
The Shrooboid Brat does not choose his own attacks in battle. Instead, he lets the Shroob audience choose who he should attack. During the battle, the Shrooboid Brat turns and faces the audience. The Shroob audience members then hold up red 'M' panels (for Mario) and green 'L' panels (for Luigi) to indicate which Bro. the Brat should attack. The Mario brother with the most number of votes is going to be the Bro. the Shrooboid Brat is going to attack. To attack, the Shrooboid Brat throws his lollipop at the chosen Mario brother.
Sometimes, the Shroob audience get tricky and hold up green 'M' panels and red 'L' panels (the reverse of what they should be). Again, the Bro. with the most votes is the one the Shrooboid Brat will attack. Even further into the battle, the Shroobs in the audience have multiple votes, meaning the brothers are attacked multiple times in the order that the votes are decided.
Occasionally, the audience gives the Shrooboid Brat items to use in battle. Again, the audience indicates which item should be used on which brother. If the Shrooboid Brat is given a Fire Flower, he shoots a fireball at the chosen Bro. This fireball can be repelled with a Hammer strike. The audience also sends either Mushrooms or Poison Mushrooms to the Shrooboid Brat. However, he spits out the opposite at the Mario Bros. (If given a Poison Mushroom, he'll spit a normal Mushroom at the Mario Bros., and vice versa.) Most of the time, the audience sends him multiple items, and he spits them up in the reverse order, starting with the last one first.
Eventually, the four Mario brothers defeat the Shrooboid Brat in combat. Princess Shroob sends Shroobs with hammers into the arena. With their hammers, the Shroobs pound a hole in the ground, causing the Mario brothers to fall into the Gritzy Caves.
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