In Mario Brothers Plumbing, Mario and Luigi are currently repairing a large, metal device; as he works on the device, Mario has Luigi give him various tools to use on the device, such as a wrench and a screwdriver. After using these tools to fix the device, Mario asks Luigi to give him a rag, which Luigi tries to do, only to grab the nearby Clog by mistake; apologizing to Clog for grabbing it, Luigi can only watch as Clog quickly scurries away.
After Clog runs away, Luigi grabs a nearby rag and gives it to Mario, who, after grabbing this rag, notices a strange man standing behind him. Proclaiming that there is a stranger in the apartment, Mario, as well as Luigi who, after searching for the stranger Mario announced, sees the man nearby. Asking who the odd man is, Mario gets the response from him that he is someone from the past, with the man, after saying this, asking Mario and Luigi if they remember him. When Mario and Luigi respond that they don't know who he is, the man begins to say that he is family and tells Mario and Luigi that he is their long-lost brother Pietro, who no one ever mentioned due to the fact that Pietro's very name would bring grief to Mama Mario. After explaining his origin to Mario and Luigi, Pietro proceeds to give them a large hug, with Mario and Luigi looking a bit shocked and uncomfortable in this hug.
Sometime later, as they eat some pizza, Mario and Luigi ask Pietro why Mama Mario never mentioned him and get the response from Pietro that Mama Mario was ashamed of him and that, at an age when Mario and Luigi were to young to remember him, he left home and traveled the world. As he begins to sob, Pietro tells Mario and Luigi that after traveling, he decided to return to his family and the plumbing business, which Pietro says he wishes to take over. While Mario is more than happy to have met Pietro, Luigi is somewhat suspicious of him and begins to ask Pietro a variety of questions, such as what Mama Mario's maiden name is and what Mario's birthmark looks like. With little trouble, Pietro answers all these questions correctly and even reveals that he knows Luigi had a teddy bear named Mr. Tough when he was small. Apparently accepting Pietro's answers, Mario and Luigi accept him as their brother; after handing Mario some more pizza, Pietro says that he wishes to sell Mario Brothers Plumbing so that he, Mario and Luigi can spend more time together.
Later, as Pietro inspects Mario Brothers Plumbing, Luigi, still suspicious of him, says who does Pietro think he is, deciding to simply sell their business. As Mario tries to reassure Luigi, saying that Pietro is their older brother, Luigi, still doubtful of Pietro, decides to call Mama Mario on the telephone. As Luigi begins his telephone call to Mama Mario, Mario learns from Pietro, who has finished his inspection of the apartment, that the fixtures of the place can be sold tomorrow, while everything else can be sold in a week's time. As Pietro tells Mario that once Mario Brothers Plumbing is sold, he will gain seventy percent of the profits while Mario and Luigi will receive thirty percent, Luigi quickly answers the ringing telephone.
As Mario and Pietro listen to him, Luigi remarks that the IRS has assessed the Mario Brothers Plumbing for $900,000 in unpaid taxes. Luigi calmly says this is not a problem, as he will pay $300,000, Mario another $300,000, and Pietro will pay the last $300,000. Pietro quickly tries to flee from Mario Brothers Plumbing, only to be stopped by Mario, who asks where Pietro is going in this family crisis. After Mario asks this, Pietro suddenly claims that he isn't actually Mario and Luigi long-lost brother, he was simply a man who, after researching Mario and Luigi, tried to swindle them out of their business for some easy money. As Pietro quickly leaves, Luigi begins to laugh, which confuses Mario, who asks Luigi why he would be laughing at a time when the plumbing shop owes nearly a million dollars in back taxes. Luigi, however, explains that the call was actually from the pizza parlor, to tell them that their pizza is ready.
After explaining this to Mario, Luigi decides to go pick up their pizza at the Pizza Parlor with Mario, claiming that they will evenly split the pizza, with Luigi getting seventy percent of it while Mario will get thirty percent of it.