Mario & Luigi need to make a little extra money to help pay for one of Mario's crimes (probably vandalism, petty theft, or property damage). Mario, of course, can't do squat on his own, so he enlists the help of Luigi to start a plumbing business. Luigi knows what he's doing, but all his work is messed up by Mario who either clogs the drain (probably with spaghetti), and/or breaks the actual plumbing.

Eventually, Luigi finally puts his foot down and tries to teach Mario plumbing. He becomes... Eh... Passable at it and they finally get the money to pay for the damages caused by Mario, only for Mario to do the same thing except 10 times worse, leading to a BONK on the head from Luigi's plunger or monkey wrench.