SMG4: Mario SAW is the thirty-sixth episode of Season 8 and the four hundred and twenty-ninth overall to be uploaded by SMG4.
The video begins with Mario waking up. He wondered if he partied really hard yesterday. Then he noticed Luigi asleep, wondering if he ate too much spaghetti last night. He then fully woke up, finding himself and Luigi in a room with a chain tied to his leg and a Piranha Plant mouth on his head. He wondered what was going on, observing the Piranha Plant mouth on his head (which he mistook for an ugly hat) and the chain that was tied to his leg. Luigi then woke up, fearfully asking whether they might die or not. Mario then told Luigi that it was probably one of his fans that kidnapped him, before being interrupted by a person wearing a Majora's Mask replica on a screen. The person asked the duo if they wanted to play a game. Luigi panicked, whereas Mario asked if they could play a particular game.
The person instead said that in thirty seconds, the Piranha Plant heads would close on their heads, although Mario ignored that and wanted to play something else. As the person tried to resume, he was interrupted by Mario a couple times over how the "game" was boring. Annoyed, the person left and the clock started to count down from 30 seconds. Luigi panicked, telling Mario to help him, even though Mario slammed himself on the wall in an attempt to get the Piranha Plant on his head off of him. 20 seconds remained, and Luigi asked the Piranha Plant to get off his head in a nice tone, but instead, the plant bit down harder on the green cap plumber, sending him into a total panic. 10 seconds remained, and Mario tried to saw the chain off his leg, while Luigi laid on the floor hopeless. 5 seconds remained, and Mario boasted that he could not die like this. Suddenly, the two Piranha Plants got off the brothers' heads and greeted each other. They then walked off, with the person wearing the Majora's Mask looking in confusion. The chain unlocked both brothers' feet, with Mario celebrating while Luigi remains in shock. Then two more chains locked themselves onto the brothers' feet, dragging them into a dark tunnel.
Mario and Luigi kept on getting dragged by the chains, entering into the Tunnel of Doom, where they noticed a tunnel filled with obstacles. Luigi managed to avoid some of the obstacles, while Mario barely dodged any of them. They then approached a Whomp, and although Luigi managed to go under it, Mario got stuck due to his obesity. The person wearing the Majora's Mask then sped up the chains dragging Mario and Luigi, resulting in the Whomp being dragged with Mario. At the end of the tunnel, the game's mastermind met them in person (only for Mario to throw things at him in anger), sending them to the next room as payback.
The Mario Bros. then found themselves in a room full of lava where they were hanging onto chains with buttons attached to their legs. The person told them that one of them needed to press the button attached to their leg, which would send them falling to their doom, while the other one advanced. If none of them pressed the button, they would both die from the rising lava. Luigi panicked over the idea, while Mario threw multiple items at the chains hanging him. Luigi eventually suggested that Mario swung over to him, although this proved unsuccessful. Luigi looked at the lava, and, with no other choice, told Mario that he was going to do the unthinkable; press the button to fall into the lava.
Mario pleaded with Luigi to not do so, but despite this, sacrificed himself, leaving his older brother into tears. Suddenly, Luigi commented that the lava tasted weird, with Mario in surprise, and then revealed that the "lava" was actually just watermelon-flavored, really warm Kool-Aid. The person then commented on the show, leaving Mario furious. He eventually drains the Kool-Aid room with a green pipe, bringing Luigi with it, and sends Mario to the "Bonus Round".
Mario then fell into a chair in a dark room and was cuffed to it. He was asked by the person with the Majora's Mask if he wanted to play a game, to which he simply answered no. The person said that the game was about Mario truthfully answering his question, which was: "Will you be my friend?". Mario obviously said no repeatedly, even though the person said "Please". The person then laughed, ultimately revealing himself to be none other than Waluigi. Mario, in total ambushed shock, instantly recognized Waluigi, who thanked Mario for rejecting him, laughing villainy, and turning into his super-power mode and teleporting in front of the red plumber, who, still in terror, claimed Waluigi had gone insane.
Waluigi replied by revealing his flashback where he sent approximately one thousand emails to the Mushroom Kingdom to come to a fake birthday party. Every single person he sent his email to had rejected it, with Luke Lerdwichagul (because his memes were bad), Wario (apparently because he had a date with some KFC chicken), Meggy (who pointed out the outcome from SMG4: Mario And The T-Pose Virus), and Bob the Builder (who simply replied no) commenting on it. For every rejection he got from every person, Waluigi became more powerful, until Mario was the sole exception as he accepted the email, commenting if there would be food there. After the flashback, Mario realized that Waluigi set up the obstacle course and made him think Luigi died just so that he could hate him. Waluigi confirmed this himself, and before he opened up a portal, he told Mario that they would meet again. Mario was left in the dark room, wondering what would happen next. He also commented that his behind itched and the episode ends on a cliffhanger.