SMG4: If Mario was in... Deltarune is the fifty-sixth episode of Season 8 and the four hundred and sixty-fourth overall to be uploaded by SMG4.
This video is also the fourteenth episode of the If Mario was in... series. It also debuts several characters from Deltarune such as Susie and Lancer and is the last video of both Season 8 and 2018. It was aired on December 29, 2018.
Mario finds him in the world of Deltarune (or Undertale 2).
The video starts off with Luke Lerdwichagul and Kevin Lerdwichagul reminding all viewers to get tickets for their January 2019 tour and to quickly get the Saiko Bichitaru Darkest Hour poster before it was gone from their shop.
The actual episode starts off with Mario driving in his go-kart while reading a book on how to drive for dummies. In the process he ran over a stop sign, a man, and a taxi cab before noticing that he was heading straight into a barricaded tunnel. He made a quick turn right, leading him onto a rural, unpaved road. This unfortunately led to a steep cliff, causing Mario and his kart to fall into another tunnel that brought him into Hometown from Deltarune.
In Hometown, Mario started causing all sorts of chaos on his kart by running over some citizens, hitting a car, and even stealing a donut from Undyne (who was dressed as a police officer), resulting in her and the police chasing him to a school. Realizing that he was now on the run, Mario broke into the school before Undyne could find him near his kart.
After entering the school, Mario decided to look for a way to escape Undyne but was forced to go the other way after seeing Baldi in the hallway. He entered a classroom and was subsequently accepted as a "foreign exchange student" by the teacher, who happened to be Alphys. Shortly after, Susie stormed into the room, at last "on time for once." However, Mario then proceeded to insult Susie by calling her "Barney" leading her to leave the classroom. He then called her a bitch, resulting in Alphys sending him out after her so he could apologize to her. In the hallway, Susie was about to beat Mario up but he managed to run away, even when Undyne had entered the school to give Mario payback for stealing her donut.
Eventually, Mario and Susie both entered a dark room where the door closed behind them. They both fell into the Dark World where Mario played with one of the local "objects" prior to stumbling upon Castle Town and the Fountain of Darkness (which Mario claimed was made of Pepsi). Excited about the presence of the fountain, Mario ran into the castle where a cloaked figure started telling him about a legend (albeit boring Mario to the point of him sleeping). The cloaked figure then encouraged Mario to join him and save the world, revealing himself to be a green guy with glasses named Ralsei. However, Mario refused to comply with him and kicked him away, causing some dark being with a spade logo to run into the Italian with a motorcycle. They passed by Susie, who was also disagreeing with Ralsei's intents for her; this caused her to want to join in on the action as well.
Later, the motorcycle-piloting figure stopped and revealed that Mario wanted to take the fountain from his kind. However, once Mario told him about the liquid coming out of the fountain, he also became excited about the fountain, causing the two to start fighting (the motorcycle being's name is revealed to be Lancer in the battle).
After Mario unleashed his "inner demons", the fight was interrupted by Susie who destroyed the "demon" with an ax. She declared that she was joining the fight due to still being pissed about what Mario did earlier, only for Ralsei to resolve the conflict by singing (albeit horribly, which fried Mario's head and set Susie and Lancer on fire).
Now with the conflict settled, Ralsei ordered the others to stop their quarreling and join him so they could "save the world". While walking towards the Card Castle (the location of the Eastern Fountain that was disrupting the balance between light and dark), the four encountered a K. Round blocking their way. Lancer explained that the checker patrolled the area as a defender of the fountain, causing Susie to take out her ax and prepare to fight it. However, Mario decided to give the beast a "Mario hat" and tame it, causing it to run into the heroes and rampage through the area. It kicked and stomped on various Darkners in the Dark World before jumping on top of the Card Castle. While doing so, the checker crushed Lancer's father (the King) and Rouxls Kaard (who was cooking macaroni and cheese at the moment), smashed through several floors of the castle, and ended up in a room, where Jevil happened to be locked up for his view of the world. Not thrilled about what happened earlier, Susie tried to leave the room but the door here disappeared as well. Asking if the heroes were prepared for his "game", Jevil proceeded to turn everything in the room into their three-dimensional forms.
In his "game", Jevil fired some explosive energy balls while evading his challengers' attacks; he even used a dagger to stab anyone who disagreed with his own beliefs. However, when he started taunting the others about being able to do anything, Mario immediately used those words to counter-attack his powers by questioning them, going as far as asking if he could spawn dangerous lethal weapons at all. Not realizing that the heroes have exploited him to their advantage, Jevil proceeded to drop a series of miniguns, RPGs, and other explosive firearms on the battlefield.
Mario, Ralsei, Lancer, and Susie then defeated Jevil using his guns and climbed out of the hole that they made with the K.Round, with Ralsei thanking everyone for their efforts in "saving the world". While Lancer and Ralsei remained at the Card Castle, Susie and Mario left and made their way to a light portal to not only return to Hometown, but also seal off the Eastern Fountain to restore balance to the Dark World. They both woke up back at the school, presumably in the dark room that they entered earlier, now littered with all sorts of games and playing cards. The two said their goodbyes in the hallway as Mario headed out of the school, even though Susie still wanted her revenge on Mario for calling her "Barney".
Finally, as Mario exited the school, he realized he was surrounded by Undyne and the military in addition to her police forces, due to his 5-star wanted level for stealing her donut.