99.5% Crazy is SMG4's 110th blooper, in which Mario eats a Poison Mushroom that causes him to go crazy.
The episode begins inside Peach's Castle, where Lakitu remarks how peaceful everything is. However, no sooner than this does Mario come running into the foyer, chasing after a plate of Spaghetti. He eventually catches the dish and eats it, when he suddenly hears the doorbell ring.
Mario goes to answer it and finds a shadowy figure hunched over and breathing heavily. He mistakenly assumes that the man is giving birth, and runs back inside to warn Toad, who orders Mario to leave him alone. Mario runs back outside and tries to help the man deliver by throwing a rock at him, but instead hits a random little girl. The shadowy figure appears behind him and explains to Mario that he is a travelling merchant. Excited, Mario asks him what he's selling.
The merchant shows off his inventory, but none of the items seem to interest Mario. He eventually pulls out a Poison Mushroom that he "found under the toilet," which Mario decides to buy to appease his appetite. The merchant charges him 10 coins, forcing Mario to leave and collect coins by plowing through enemies on a Koopa Shell. He returns some time later and literally throws money at the merchant before grabbing the Poison Mushroom and running back inside the castle.
Mario excitedly runs into SMG4's office and eats the Poison Mushroom, despite Lakitu's protests. He initially feels fine after eating it, when he suddenly falls under the mushroom's influence and into a hallucinogenic state that causes him to go insane.
Outside the room, SMG4 calls out to Mario, demanding to know if he shoved spaghetti into his computer. Mario suddenly bursts out of his office and starts frantically running around the foyer, even attempting to fly by jumping across the room. Horrified, SMG4 asks Toad what happened to Mario. After the Toad loosely describes how Mario bought a mushroom from "a guy having babies," SMG4 runs out of the castle to check on his friend.
Outside, SMG4 finds Mario standing completely naked on top of the castle roof, declaring himself the "King of the Spaghetti Land." He begs Mario to come down and asks him if he even remembers SMG4's name. Mario's brain goes into "Processing Mode" (in the same action as a dial-up computer), but his mind is so broken that all he does is explode and fall to the ground. Growing even more concerned, SMG4 decides take Mario to the hospital.
Later, SMG4 and Mario arrive at the Crap-E-Hospital, where the latter decides to barricade the entrance to fight off against the "old, scary, sexy people." While SMG4 tries his best to explain the situation to the receptionist, Mario gets into a fight with a vending machine after he mistakes it for a toilet and accuses it of stealing his money. They are later escorted to the doctor's office, only to find said doctor absent; the reason being that he is playing Tetris in the bathroom.
Dr. Mario finally arrives some time later, and SMG4 tells him that something is wrong with Mario. The doctor initially believes that the problem with Mario is his obesity, but SMG4 corrects him by stating that the problem is mental, which Mario demonstrates by randomly walking and falling off of the examination table. Although Dr. Mario doesn't see the problem, he still gives loose diagnosis that the plumber either has a rare disease or is highly intoxicated. Disappointed, SMG4 leaves with Mario, who he finds sleeping with his eyes open.
On the way back to the castle, SMG4 contemplates over the idea of living with a Mario that is even more stupid than before. However, when he finally gets home, he looks behind him to find that Mario has completely vanished.
In another part of the kingdom, Mario wanders around aimlessly in a nearby town, disturbing the local residents with his antics. He shoots Wario when he tries to mug Mario, and even starts crawling around backwards.
Back at the castle, SMG4 tries to use a GPS to track down Mario, which doesn't work. Frustrated, he throws the device away, accidentally hitting Luigi in the process. However, he doesn't care and instead worries about whether or not Mario is hurt or dead.
Meanwhile, Mario resorts to frantically running around in circles and making noise, which ends up annoying a bystander. When he tells Mario to stop, the plumber just decides to run around a bit further away, annoying the man further.
SMG4 starts searching the entire kingdom for Mario, even accidentally peeping in on Old Man Hobo taking a shower.
Back in the village, a robber confronts Mario and demands for his wallet. However, Mario does not understand and offers a bunch of random items instead, including another Old Man, a bag of Doritos, and a dancing Luigi. Annoyed, the robber gives up and leaves Mario alone, realizing that the plumber is too stupid to rob.
SMG4 suddenly arrives at the village and is relieved to see Mario (who calls SMG4 his father). The two of them dramatically run towards each other, but instead of embracing, Mario runs right past SMG4 and hugs another bystander. Annoyed, SMG4 takes Mario back to the castle, the latter of which follows him while continuing to move sporadically.
When they finally arrive at the castle, SMG4 confronts Mario, stating that there needs to be some new rules since he can't fix his friend's mental state. He decides to make Mario his "Nacho Slave," but the plumber seems confused about what that means and wanders off. When SMG4 catches him, Mario suddenly pulls a gun on his friend, demanding be the Spaghetti King again. Terrified, SMG4 tries to take back what he said, while Mario demands that he make him spaghetti and announces supposed plans for world domination.
SMG4 runs inside the castle, begging Toad to help him. However, the mushroom being still doesn't care. Worried, SMG4 tries to think of what he can do to stop Mario, when Wizard Rock (Rocks) suddenly appears in the foyer. SMG4 explains the situation and asks the magical rock if he can help. Wizard Rock tells SMG4 that the only way to save Mario is to defeat him in a Magic Duel, and gives him a magic scepter that has been in the Rock family for generations.
SMG4 heads outside and finds Mario back on the roof, just as he was before. He challenges the plumber to a Magic Duel, and the two of them are teleported to a small arena. SMG4 pulls out the scepter to begin the battle; but when he tries to cast a spell by throwing the scepter at Mario, it flies right past him and hits Luigi off-screen (again). Mario then gains the upper-hand and defeats SMG4 by making his legs disappear.
After the duel, SMG4 crawls back into the castle, where Wizard Rock him asks how it went. SMG4 admits that he lost and that he lost the wizard's scepter. However, Wizard Rock isn't too worried, as he can just buy another one at Toys-R-Us. Mario suddenly barges into the foyer, telling SMG4 that he plans to kill him and put him in "Mario's Tunnel of Doom." SMG4 runs away screaming while Mario laughs maniacally.
Suddenly, a 1-Up Mushroom appears out of nowhere and comes into contact with Mario, curing him of his insanity. Finally sober, he asks what's going on and is confused when SMG4 suddenly tackles him. Mario explains that he isn't crazy anymore thanks to the 1-Up Mushroom, and SMG4 is finally able to relax. Wizard Rock narrates the ending for the episode, saying that the two friends finally returned to normal and "[had] many babies after" (much to Mario's annoyance). However, the episode ends on a fake cliffhanger, as Luigi has somehow gone insane as well.