Junker is encountered while infiltrating Peach's Castle Garden near the end of the game. Upon entering the back area of the garden where the secret entrance was supposed to be, Starlow comments that there is nothing but trash. After Mario and Luigi progress to the end of this area, Midbus appears, holding some sort of remote control. When it is activated, Junker springs to life from the piles of garbage laying around the area. After Midbus's attempts to control it fail, he smashes the remote in a rage and allows it to fight the Mario Bros. on its own. Midbus's attempt to control the Junker may demonstrate why it was in the garbage in the first place, as it began to dance when he commanded it to attack.
In battle, Junker has a variety of attacks. One attack consists of throwing a bucket over either Mario's or Luigi's head. The brother then stumble around blindly while Junker throws Thwomps at him; players must time their jumps to avoid the attack. If Mario or Luigi are hit by the Thwomp, they may get dizzy. This attack is a reference to the Super Famicom game Mario & Wario.
Another attack involves throwing two balls of metal scrap at one of the Mario Bros., then attempting to ram him. The brother in question must jump onto the balls, then jump over Junker as it attempts the ramming portion of the attack. A third attack consists of emptying the garbage from one of the Junker Cans that follow it. Often, this garbage consists of random enemies that go after the Mario Bros, which include:
Once several enemies have attacked, the Junker Can is discarded. If Mario or Luigi defeats a Junker Can, a Chain Chomp or a Fishbone may appear. The Fishbone circles Mario or Luigi, and if not hammered, rams into them. The Chain Chomp charges at one of the brothers, who must counter it with the hammer. The Chain Chomp can also inflict poison.
The Junker can also choose to hinder the Mario Bros. and incapacitate Luigi by putting him into a Junker Can. Since this attack cannot be dodged, Mario is forced to defeat the Junker Can to rescue his brother. Occasionally, Luigi hits the rim and escape on his own; on very rare occasions, Luigi waves to Mario while in one of the Junker Cans. After doing this, the Can's lid slams on Luigi's hand. If he is not freed and Mario is defeated, the player receives a Game Over (assuming they do not have any Retry Clocks).
After the battle, Junker begins to malfunction again, rampaging around and freeing Bowser from the safe he was trapped in earlier. It then destroys itself by running into a locked gate, opening said gate in the process. This allows the Mario Bros. to access a pipe connecting Peach's Castle to Pipe Yard.
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