Extra: HEY! Butts!

Extra: HEY! Butts! (外伝：HEY！尻！) is the first OAD of the anime adaptation of That Time I Got Reincarnated as a Slime.

Synopsis
Mikami Satoru, an ordinary businessman, was stabbed on the street and died, but found himself reincarnated in a parallel world... in the body of a slime! Thus he began his new life as a slime named Rimuru, flung into a new world teeming with creatures of many different races, and made the decision to start a new country where all races could enjoy life together. In the midst of his everyday life, Rimuru suddenly realizes that there's nothing to do for recreation in Tempest. He suggests sumo wrestling as a way for everyone to have fun together, and his friends are immediately on board with the idea. Thus begins Tempest's first sumo tournament, in which everyone competes enthusiastically in hopes of winning the grand prize! Passionate contests break out not only among Benimaru and the men, but also among women like Shuna and Shion, and even Rimuru ends up joining in! This is a special episode in which all the citizens of Tempest come together for some fun.

Summary
Gabiru and his personal overzealous guards, tend to the Hipokte Grass within the cave the Lizardmen of Tempest inhabit. Deeper in the cave, Rimuru requests Vesta to help him with his sleeping problem and the former minister, prescribes some sleeping pills. At the safety of his home, Rimuru takes a couple, aware that the pills are in the beta stages, but is confident that the Great Sage will help him if he's in trouble. Taking the pills, Rimuru becomes drowsy and dazed until he finally passes out.

Awakening in front of Shion and Shuna, they have Rimuru try on an assortment of different clothes. As this goes on, Rimuru realizes that Tempest doesn't have anything fun or entertaining. So, he decides to introduce sports into the city, starting with Sumo Wrestling. A formal announcement about the sport is made to the citizens and Rimuru explains the rules and regulations behind the sport. Though, it's noted that the citizens have trouble with comprehending said rules and regulations with their violent tendencies.

Eventually, Myrd sets up a couple of sumo rings and Garm with Geld show off the traditional Sumo attire, the Mawashi. All of the other participants dress in Kimonos and Rigurd explains the prizes for winning the Sumo contest. On the ladies side, it's to spend a day with Rimuru, opposite to that, the guys side is to be able to use Shuna's lap as a pillow. Most notability the men side, are fired up and Ranga shows off his clothing attire that makes him look like a Tosa. Soon the matches starts with Geld vs. Kurobe and with Hakurou acting as the judge. As that goes on, Rimuru checks on the ladies side and sees Shion and Souka in their personalized Mawashi. Realizing that there's no judge for the female side, Souei volunteers for the position; the reason being that Benimaru easily overwhelmed the male side of the competition.

The first match to the ladies side, starts with Shion vs Souka, and due to a wardrobe malfunction by Shion's horn, Souka forfeits. Just as Treyni joins as a spectator, the ever pompous, Milim steps into the ring and faces off against Shuna. Because of Milim's immense strength as a Demon Lord, a couple of restrictions are placed on her, to which, she accepts. The match then begins and in the end, Shuna wins due to an indecent act by Milim. With the ladies ring now destroyed, Rimuru suggests they use the guys ring, however, that ring is being used by the Goblin children. Therefore, Rimuru changes the final match with Shion vs Shuna be a game of Butt Sumo. At the start of the match, it gets mildly erotic, due to the nature of the sport. In the end, however, due to the potent power of Kijin, the ring crumbles causing Rimuru to intervene, sequentially ending the match in a draw.

From there on out, Butt Sumo became the national pass time of Tempest. At that moment, the surrounding area around Rimuru changes into a post-apocolyptic wasteland because of Shion and Shuna's Butt Sumo match. They then grab onto Rimuru in his Slime form and start pulling on him each wanting him for their own. Before Rimuru was torn apart by the two Kijin, he awakens in his room and Great Sage informs him that the effects of the sleeping drug has warn out. Returning to Vesta, Rimuru has him lock the remaining sleeping pills away forever, due to the potency of the product.

Characters
MAIN: BRIEF APPEARANCES/CAMEO: RETURNING CHARACTERS:
 * Rimuru Tempest
 * Great Sage
 * Veldora Tempest (Mentioned)
 * High Orcs
 * Rigur
 * Hobgoblins/Goblinas
 * Dragonewts
 * Goblins
 * Tempest Wolves
 * Shizue Izawa
 * Rigurd
 * Gobta
 * Ranga
 * Dord
 * Myrd
 * Vesta
 * Benimaru
 * Shuna
 * Shion
 * Souei
 * Hakurou
 * Kurobe
 * Treyni
 * Gabiru
 * Souka
 * Geld
 * Milim Nava

Locations

 * Sealed Cave
 * Jura Tempest Federation
 * Rimuru City

Trivia

 * This OAD is bundled with the special edition of Manga Volume 13.
 * This episode was originally scheduled to release on March 29, 2019, but was delayed to December 4, 2019.
 * When Rimuru was thinking what fun things he can bring to Tempest, the things shown he thought of include various Light Novel, Manga, and Anime DVDs of the TenSura series as well as some well known famous video game consoles.