The Plumber's Oath is an item in Brain Drain. It is a "secret" that all plumbers must apparently follow. When Mario and Luigi are discussing how they can recognize each other if their minds are swapped by Iggy Koopa's Synapse Switcher, Mario pulls out a piece of paper with the Plumber's Oath. He tells Luigi that if they memorize the oath, they can recognize each other no matter what. The paper is missing the oath's last line, though Mario assures Luigi that any decent plumber can figure it out for themselves; to the reader, this means solving a puzzle. Later in the book, Luigi's mind is swapped with that of a Koopa Troopa, and when he attempts to reunite with Mario, Mario asks him for the Plumber's Oath. If he misremembers the last line as "He always has a wrench handy," Mario starts to launch fireballs at Luigi until he ends up destroying the Synapse Switcher, which leaves Luigi permanently stuck in the Koopa's body and results in a Game Over. If he correctly says that the last line is "He always carries a plunger," the two excitedly exclaim each other's names before continuing the attack on the Koopa Command Center.
A plumber knows how to unclog a drain