Mario and Luigi are at home in Mario Brothers Plumbing, where the two of them are forcing a strange Slime Ghost back into one of Edison's sewer entrances with a broom. As soon as they succeed in forcing the Slime Ghost into the sewer, another larger Slime Ghost slides across the floor. While Luigi directs the Slime Ghost into the back room, Mario mentions that the Slime Busters should be arriving at any moment. As a van drives past the basement window and parks, Mario tells Luigi to keep the Slime Ghost busy, and figures that the Slime Busters have arrived.
Just as the Slime Ghost pulls Luigi into the back room, the doorbell rings, and Mario tells the person on the other side to enter. In walks Ernie Hudson, but Mario is disappointed that none of the other Slime Busters came. Ernie confesses that when the Slime Busters heard that Mario and Luigi had called, he lost a coin toss, implying that none of Slime Busters really wanted to come.
Ernie notices some steam coming from the furnace, and goes to inspect it, but Mario redirects him to the back room. Before entering, Ernie tells Mario that if he's not back in ten minutes, to put a dime in the parking meter for him, to which Mario complies. Ernie enters the back room, and comes out disgusted, complaining that what he saw in their was the most hideous entity he's ever seen. Soon, Luigi walks out of the back room, screaming and shaking. Mario asks Ernie to introduce himself to Luigi and to shake his hand, but Ernie comments that Luigi is shaking just fine.
Later, Ernie is checking the reading on one of his devices, noting that he can't pick up a trace of the Slime Ghost. When Mario suggests that it went for a bite to eat, Luigi, who is sitting at the table, says in creepy voice, that a slice of pizza is his favorite dish, and proceeds to eat the plate rather than the actual slice. After witnessing some slime being flung onto the fridge from nearby, Ernie claims that the Slime Ghost is indeed nearby. Luigi, now with a slimy, glowing, half body, startles Mario and Ernie, and offers them a chance to arm wrestle his slimy arm before Mario say "uh-oh".
The Slime Ghost, having taken control of Luigi, tells Mario that he's his next target. Ernie prepares to blast Luigi with his proton gun, explaining that his shot will drive the Slime Ghost from Luigi's body, or else Slime Ghost turn him into a drooling, moronic zombie forever. Ernie fires his gun at Luigi, who attempts to deflect it with a pizza pan. After a flash of light, the Slime Ghost has diappeared, and Luigi appears to be back to normal.
Ernie asks Luigi if he's okay, and he responds, saying the good news is that he's no longer a drooling, moronic zombie. Mario tells Luigi that he can say that again, and he does. Ernie asks Luigi what the bad news is, and Luigi points to the now dented pizza pan, stating that they'll need to buy a new one. Suddenly, the pizza pan starts to glow green. From it, the Slime Ghost's voice tells Ernie that he surrenders, and asks to be taken away from Mario and Luigi, whom he refers to as "two Goombas". Upon hearing this remark, Mario and Ernie laugh.