Extra: The Tragedy of M?

Extra: The Tragedy of M? (外伝：Ｍの悲劇？) is the second OAD of the Anime adaptation of That Time I Got Reincarnated as a Slime.

Synopsis
One day, Shuna and Shion are fighting over Rimuru's squishy slime body as usual. Rimuru figures that giving them each a body double of himself would solve the conflict, but when he imagines some of his other friends asking him to do the same for them, he quickly drops that plan. Then he comes up with the idea to make a cushion in the shape of his slime form. He travels around Tempest to see if his friends know of any good materials he can use, and he learns that a special type of sand found along a lakeshore in the forest is exactly what he's looking for. So he sets out to collect some sand... but a monster is waiting for him by the lake! What's more, Milim hears about his plan for a picnic and comes racing down from the sky! The fun, relaxed story features the all-star cast of Jura Tempest Federation citizens enjoying a picnic by a lake, with never-before-seen scenes brought to you by the same cast and staff as the TV series!

Summary
Rimuru finds himself in a familiar situation, a tug-of-war match between Shion and Shuna as they compete to hold onto him. Deciding that he needs to create a substitute to make the two leave him alone Rimuru wracks his brain for answers and decides on 'the chair that makes people fall into depravity' a bean bag chair made in his image. After questioning Shuna, he learns that Spandex has already been invented in this world, what he's missing is the filling.

Consulting with Garm, Rimuru learns that the material that he needs, Sarasa, is located on the sand bar of a lake within the Jura forest. Which leads to Rimuru holding an expedition/picnic there. However, Garm also warns that a monster is living in the lake, albeit one probably not as strong as Rimuru. Rimuru explains his plan to Benimaru, Souei, and Gobta, upon learning of it Benimaru treats the objective as a number one priority.

If Shion is given a Rimuru cushion by Rimuru himself then it would become her prized possession and such a huge priority for her that she never steps foot into the kitchen again. Overhearing their conversation Hakurou states that he will also join in. The Expeditionary force has a safe and passive trip to the lake and spots their objective in the middle of the lake. Before they go to collect it however they decide to eat first.

Gabiru eats the food his sister Souka made for Souei. Shion's cooking manages to exceed all expectations: her latest concoction eats through the pot and burns through the ground. She then presents Rimuru with rice balls, which appear to suffer from some kind of demonic possession, much to Rimuru’s disgust. The balls are thrown into the lake as Shion falls into the hole made by her food, only to get stuck because of her breasts. Gabiru is then beaten up by Shion for not stopping them as he was disgusted by their appearance. Deciding to move on with their day Rimuru has everyone change into the swimsuits that the Dwarfs prepared for them.

The girls all receive flattering two-piece swimsuits, the men get speedos, while Rimuru and Ranga receive feminine one-piece swimsuits, leading Rimuru to simultaneously praise and curse the dwarfs who made them. Before they actually go into the water Milim arrives and demands to know why Rimuru didn't invite her to something as fun as a picnic and swimming in the lake to which Rimuru claims that he thought she was too busy.

With everyone dressed they proceed to swim out to the sandbar but on the way, they are grabbed by tentacles. Investigating the source Rimuru discovers the rumored monster of the lake, a giant shelled cephalopod which is lashing out blindly at its surroundings. Investigating further Rimuru discovers the creature is having a painful reaction to Shion's demonic rice ball. Having figured out the source of the problem Rimuru throws many health potions on the creature curing it of its food poisoning.

Rimuru then teaches the monster Magic Sense granting it sight and allowing it to perceive it's surroundings, the monster's situation reminding Rimuru of his situation before he met Veldora. Swimming to the sandbar with the monster, Rimuru learns that the monster had an effect on everyone: their swimsuits are literally falling off of them, dissolved away by the monster. Rimuru explains the situation to everyone, destroying the demonic rice ball in the process.

According to the Great Sage's analysis, the Sarasa is sand that has absorbed a large amount of water and then refined into the smooth squishy material they came there for by the monster's mucus. With this in mind, Rimuru plans for several return trips to the lake both to visit the monster and collect more of the sand. As for the Rimuru shaped bean bag chairs... the girls have completely ignored them in favor of fighting over Rimuru again.

Characters
MAIN: BRIEF APPEARANCE/CAMEO: RETURNING CHARACTERS:
 * Rimuru Tempest
 * Great Sage
 * Satoru Mikami
 * Hobgoblins/Goblinas
 * Gobichi (Mentioned)
 * Garm
 * Dord
 * Myrd
 * Veldora Tempest (Mentioned)
 * Rigurd
 * Gobta
 * Haruna
 * Ranga
 * Benimaru
 * Shuna
 * Shion
 * Souei
 * Hakurou
 * Gabiru
 * Souka
 * Dragonewts
 * Milim Nava

Locations

 * Jura Tempest Federation
 * Rimuru City

Trivia

 * This OAD is bundled with the special edition of Manga Volume 12.
 * The girls' swimsuits each suit them in different ways: Shuna's is pink with frills, a typical representation of youth; Shion's suit is more "adult;" Souka's is more sporty and form-fitting, along the back her top allowing for the use of her wings; Haruna's suit resembles her outfit from when she was a goblin.