SMG4: Freddy's Ultimate Custom Spaghetteria is the twenty-ninth episode of Season 8 and the four hundred and twenty-second overall to be uploaded by SMG4.
Yeah. They're back.
Mario wondered what happened to SMG4 and the cause of his rage when he realized where SMG4 put him: back in the security room of Freddy Fazbear's Pizza. Hoping there was no animatronics nearby, the phone suddenly rang, freaking him. Attempting to shoot the phone failed as the receiver worked fine anyway. Suddenly, the animatronics got close to him. After a few humorous moments with them, Mario managed to find an air vent to escape.
After reaching the other side of the air vent, he came across Balloon Boy playing Minecraft. Not wanting anyone else to find out, he somehow merged with him, tricking Toy Freddy (who was aware of Balloon Boy's commotion) into thinking he heard nothing. Staying in disguise, Mario looked around to see some animatronics in a singing contest. Its contestant, Withered Chica, was giving a very poor performance. Using the situation well, Mario found another exit.
After that performance, Mario attempted to unlock the door but to no avail. The next contestant, Mangle, gave out an ear-piercing screech as his singing. After his performance (where the audience was on fire), he asked if he did well. When no one gave their response, he angrily asked again, causing everyone to celebrate it out of fear. Just as Mario was trying to escape, Mangle asked how he did, which Mario responded by saying he needed help for it. Triggered by such bad review, Mangle attacked him.
When Mario woke up, he found himself as his animatronic self. Shocked at the revelation but pleased there was no other animatronic nearby, he explored the area just to bump into Phantom Freddy. They both screamed at each other in terror before Phantom Freddy blew up. That attracted Springtrap to investigate but when he saw Mario, he became infatuated with him. Springtrap tried proposing to Mario but he was rejected. Angry, he slapped Mario through a wall (somehow returning him back to normal) where he soon fell unconscious.
Mario woke up in a bed confused. As soon as he thought, the Nightmare animatronics appeared ready to spook him. After feeling shocked by them, Mario deduced that if kept his eyes closed for a while before reopening them, they would disappear. He was right and the bedroom was deserted. Soon, it was populated by his friends where Luigi right in front of him declared that he was going to eat his spaghetti for him, Fishy Boopkins watching some Five Nights at Freddy anime and inviting Mario to come and watch it with him, Bowser declaring that Toad was his new best friend, Meggy saying that he got replaced from her Splatfest team by Baldi wearing a Mario face, SMG4 asking if he had exercised, and when he decided to hide in a room, Bob telling him he got a nice a** in the room.
Fortunately for Mario, it was all a nightmare and when he woke up, he found himself in an elevator in Sister Location, hopeful it could take him out of the place.
In the elevator, HandUnit appeared and introduced himself, asking Mario to choose a piece of elevator music. When Mario asked what was going on, HandUnit chose Çasual Bongos, causing Donkey Kong to appear playing his bongos. HandUnit then asked Mario to type out his name. As Mario typed M, HandUnit quickly concluded that his name was ThiccBoi. HandUnit then shoved Mario into a workroom and began explaining what he needed to do, just for Mario to take away an air vent covering, throw it at HandUnit and flipped him off as he went through the air vent.
As he entered the vent's other side, Mario found himself in pitch black except for a lit door. Just as he was about to make his way to the door, he heard a piece of music that could sound like a Harry Potter music. Deciding to use his "Super Sneaking Technique" (it involves him in a trash bag), he made his way across but was caught anyway by Ballora. Ditching the technique, Mario ran for the door. However, before he could reach it, the door's light closed once again and in its place stood Circus Baby, who asked why Mario was here and thought about the reasons.
Mario told her to "shut the f*** up", triggering her to use a metallic claw from her chest to pull Mario in. Just as all was lost for him, he was saved by a group of sentient garbage collection called Trash and the Gang, who mistook him as one of their own due to his disguise. As Circus Baby was attacked by them, Mario used that opportunity to escape but slid down a chute where he ended up crashing an animatronic band performance.
When Mario came to, he realized with horror that the entire animatronic gang was after him for ruining their entertainment. Just they were about to attack him, they suddenly froze due to it being 6 AM.
SMG4 then reappeared, saying that he could no longer tolerate the video any longer and happily asked Mario how he felt, which he could only cry in fear. He then announced that he would no longer do any more FNAF videos just for Lakitu to whisper something in his ear. SMG4 then changed his mind due to Scott Cawthon announcing seven more FNaF games (non-existence in real life), where Mario could only scream in terror.
NB: Knowledge of FNAF music may be useful for this list