After being taken into Rimuru's body, Ifrit awakens to find the Storm Dragon Veldora standing before him. Veldora calls Ifrit to where he is, delighted to find a new conversation partner. Thus the two of them begin to watch over Rimuru's actions from within him and recaps events of the season while in a discussion with Veldora as they play Shogi.

One day, when Shuna and Shion are fighting over Rimuru's squishy slime body as usual, Rimuru 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 lake shore 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!

Being physically incapable of sleeping, Rimuru takes an experimental sleeping pill Vesta invented, and dreams about his attempt to make Sumo Wrestling Tempest's national sport.

Rimuru, who is teaching Shizu's students as a substitute at the Kingdom of Ingrassia's Freedom Academy, takes part in an orientation in actual field combat in the school's annual outdoor training event. Transfixed by the prize he gets if he wins, he accepts a challenge by the teacher Jeff. However, an enemy appears before the students...

Rimuru, now a teacher at the Freedom Academy in the Kingdom of Ingrassia, has become involved in the school's outdoor training event with his class. The outdoor training event is an annual competition in which the students of each class guard the teachers on a journey to a nearby town. Rimuru is assigned to Class A, while his own students in Class S escort the young Tiss-sensei. All the students are eager to show how much their daily training has paid off, but the situation turns around when they're attacked by thieves!

Rimuru and the Class S students have made it through the first exercise of the Freedom Academy's annual outdoor training event. Class S has arrived first at the destination, the mansion of Count Guratol, and now they're ready to begin the second exercise, a cave exploration... but awaiting them there are thieves who hope to kidnap the students and make a fortune returning them for ransom money. Just when it seems like Rimuru's students have fought them off thanks to the skills he taught them, they realize a monster is lurking among the thieves! This is the final installment in the brand new episode created by the original author, Fuse-sensei, just for this OAD release!

A merchant from the east brings Hinata Sakaguchi a new book. In it she finds surprising information about the Jura Tempest Federation, a nation of monsters. As a former student of Shizu's, what goes through Hinata's mind upon learning about the existence of Rimuru? This special episode gives some hints as to what's coming in the new arc, as well as a look back at Rimuru's exploits thus far.

Recap of events from season two, episodes one through twelve.

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Yuuki asks Rimuru to investigate the Kingdom of Coleus, where a battle for succession has pitted the kingdom's two princes against each other. Rimuru launches a covert operation to sneak into Coleus and find out what's driving this conflict.

Aslan visits Rimuru to thank him for saving his sister, Zenobia. They discuss her illness and Souther's behavior, and begin to suspect that the court physician, Gustav, is behind both. Rimuru dons yet another disguise to investigate further.

Rimuru confirms that it was Gustav who cursed Zenobia with illness. It turns out he and his accomplice, Carl, are Archdemons plotting to manifest their god using Zenobia as a vessel. Can Rimuru stop them before the Kingdom of Coleus is lost?

After defeating Clayman at Walpurgis, Rimuru officially became a Demon Lord and a member of Octagram. Meanwhile Diablo awaits Rimuru's return in Tempest, and is anxious to learn about his master's activities as soon as possible. A look back at Walpurgis.