The episode begins at Peach's Castle, where Mario is watching TV in the foyer, while Luigi is playing with dolls. Toadsworth suddenly walks into the room, arguing wtih someone over the phone about leaving no witnesses and "[hiding] the bodies." He is suddenly interrupted by his watch beeping, reminding Toadsworth that he needs to pick up his grandson, Frankie, from school. Already having to much time on his hands, he asks the Mario Bros. if they'd be able to pick up Frankie for him. However, Mario and Luigi are both occupied with their current activities. Annoyed, Toadsworth gets their attention by breaking the TV and throwing away Luigi's dolls. He asks them once again to help him, but Mario refuses out of anger for the TV being destroyed; and Luigi is completely distraught over his dolls. Toadsworth finally decides to blackmail Mario by threatening to lock up the castle refrigerator. This is enough to get the overweight plumber moving, as he grabs Luigi and runs out the castle.
Later, the Mario Bros. arrive at Frankie's school just as classes start letting out for the day. Luigi looks at a photo of Frankie so they know where to find him. However, he is shocked to find that there are multiple Mushroom children that look the exact same as Frankie (to which Mario accuses Luigi of being racist after saying it aloud). Mario repeatedly calls out Frankie's name (which comes out as "FRANKUUU") to lure the child out. Frankie eventually hears Mario and runs over to meet the brothers. Just as they're about to leave, however, Luigi points out that the ATV bike they arrived on only fits two people. To "solve" this problem, Mario decides to tie Frankie to the handlebars and proceeds to drive off.
On the way back to the castle, Luigi frantically warns Mario not to drive so rough out of fear over Frankie getting hurt. Mario insists that the Mushroom boy is fine, only to repeatedly ram Frankie into multiple objects on the street. They eventually drive past an ice cream truck, which gets Frankie excited. Mario initially refuses to buy him any ice cream, but reluctantly gives in when Frankie starts throwing a tantrum.
Both Mario and Luigi go the truck to order Frankie's ice cream. However, when they return, they find that Frankie somehow managed to free himself from the ATV and has disappeared. They frantically search for him and hear the ice cream truck drive away. As Mario and Luigi look in its direction, they spot Frankie on top of the truck and immediately give chase on their ATV.
They manage to catch up to the truck (but not before Mario wipes out a nearby car of Toads that wanted ice cream) and try to alert the ice cream vendor, who doesn't understand and merely waves at them. Mario takes this as an insult and jumps onto the truck's windshield, causing the vendor to swerve and crash into a parked police car. Frankie ends up being the only passenger who is completely unharmed; thus the angry police officer blames the Mushroom boy for the crash and arrests him.
Later, at the police station, Frankie is placed in a cell while Mario and Luigi argue about what to do. Mario suggests buying a replacement child on eBay for 20 coins, but Luigi reminds him that Toadsworth will kill them if he finds out about Frankie. Mario insists that they'll get the boy out without Toadsworth knowing. Just as he says this, however, Toadsworth suddenly calls Mario on his cellphone. He tells that Mario Bros. that he's tired of waiting, and is on his way to pick up Frankie himself while he's on break. Worried, Mario sends Luigi out to keep Toadsworth distracted while he tries to find a way to get Frankie out of prison.
Mario walks up to the Teletubbie officer (Po) at the front window and tries to bribe his way in by completely stripping down. However, Po doesn't buy into it and points Mario to a bulletin board stating that the bail fee is 50,000 coins. Obviously not having that kind of money, Mario tries to pay the officer in Tic-tacs and Rice Krispies, only for Po to fall silent.
Meanwhile, Luigi meets up with Toadsworth and suggests that he stop and take a rest. When Toadsworth insists that he needs to pick up Frankie, Luigi tells him that the Mushroom child is in the Craft Store next to them and quickly runs inside. He frantically tries to build a decoy of Frankie using various objects in the store, and comes back out with a hodgepodge-looking mannequin. Even so, Toadsworth completely falls for the disguise and greets his "grandson", with Luigi holding the decoy up and providing the voice. Glad that they're doing okay, Toadsworth decides to take "Frankie" and Luigi out to lunch.
Back at the police station, Mario continues to bribe Po to get the real Frankie out, but the Teletubbie officer turns down all of his offers. As a last resort, the plumber attempts to use a "Get Out of Jail" Monopoly card, but Po turns that down as well. Finally fed up, Mario decides to kick down the side door and proceeds to taunt the Teletubbie. However, all this does is get him thrown out and banned from the station.
Refusing to give up, Mario climbs onto the roof of the station and tries to drill his way in on top of where his thinks the cells are. However, he instead lands right back near the entrance where Po is waiting for him. Mario thinks fast and kicks the Teletubbie officer in the groin, which gives him enough time to run further into the station.
Sneaking his way through the halls, Mario manages to knock out another officer and steals his uniform so he can get past the security cameras. He enters the elevator and tries to act natural around the other officers inside by dancing, which only makes them uncomfortable. He finally makes it to the containment area and spots Frankie in his cell.
Mario tries to break down the door using a multitude of weapons, as well as dynamite and the magic words "open sesame," but the door remains intact. Frustrated, Mario thinks about what SMG4 would tell him in this situation. When this figment in his imagination tells him to "use [his] head." Mario takes the advice literally and head-butts the door, which does break enough of it off for Frankie to squeeze right through.
Back at the entrance, Po has recovered slightly and activates the alarm. As Mario and Frankie try to escape, the former begs Luigi to hurry up and help them.
Meanwhile, Luigi is at a restaurant with Toadsworth and the decoy Frankie, where he is still speaking for the dummy to avoid raising suspicion.
As Mario and Frankie exit the police station, they are suddenly swarmed by an entire fleet of police officers. Mario grabs Frankie and runs off, with the police chasing after them with guns, cars, and helicopters.
Nervous, Luigi tries to reassure himself that Mario will fix the situation, when he suddenly sees a news story on the restaurant TV about his brother and Frankie being chased down by the cops. He throws the decoy at the TV and breaks it to keep Toadsworth from noticing. However, the chase suddenly makes its way past the restaurant, and Luigi decides to take Toadsworth home.
As their walking down the street, the chase continues right in front of them, and Luigi tries to lead Toadsworth in the other direction. They eventually come across a train station and get on.
At the same time, Mario and Frankie come across the same train and proceed to jump onboard. As Mario sits down, he spots Luigi sitting right next to him, and the two brothers quickly swap Frankie with the decoy. Mario throws the dummy off the train, and the police proceed to wipe the fake Frankie out with a tank and arrest it.
Eventually, the Mario Bros., Toadsworth, and Frankie all get off the train, and Toadsworth thanks the brothers for taking care of his grandson. He asks them if they can pick up Frankie again next week. Horrified, Mario and Luigi both refuse and run off screaming. The episode ends with Toadsworth just staring in their direction, confused by their reaction.