The Demon Lord Carrion has sent one of his Three Beastketeers, Phobio, on a raid of Rimuru's city. However, Phobio is quickly defeated when Milim meets his attack with her own. Rimuru and his subordinates then ask Phobio why he has come to their town.

Rimuru sends a delegation led by Benimaru to the Animal Kingdom of Eurazania, ruled by the Demon Lord Carrion, also known as the Beastmaster, to establish diplomatic relations after becoming indebted to him in the battle against Charybdis. A delegation from Eurazania is set to arrive in Tempest around the same time, but the Animal Kingdom is a very warlike nation, and Shion is enraged when one of the delegates, Suphia of the Three Beastketeers, makes a belittling remark toward Rimuru. The goal was a friendly relationship, but it takes no time at all for a fight to break out...

Despite the initial conflict, Eurazania and Tempest have deemed each other to be worthy of establishing friendly relations, and diplomacy between them officially begins. Tempest agrees to share its industrial techniques in return for Eurazania's agricultural techniques, and Rimuru decides to use the apple brandy produced as a result of this exchange to seek friendly relations with the Armed Nation of Dwargon next. He's accompanied by Shuna and Shion along with the former residents of Dwargon, Kaijin and the three Dwarf brothers, who are welcomed back to their home nation as state guests.

Rimuru completes the two very important duties of a discussion with King Dwargon and a public speech announcing the two nations' friendship to the people. Now it's time to head out for a little fun at the paradise he promised Gobta... while making sure Shuna and Shion don't notice.

Tempest is preparing a feast in celebration of Rimuru's return home. Then a newcomer, the majin Myulan, arrives to scope out Tempest on the Demon Lord Clayman's orders, having joined up with Youm's crew as their military advisor in order to enter the nation. Upon arriving, she's surprised to see what a strong nation it is, but continues to talk with its citizens to learn more. Meanwhile, the king of Falmuth, who sees Tempest as a nuisance, conspires with the Western Holy Church in a plot to invade the monster nation.

Following Clayman's orders, Myulan ignores the words of Youm and Grucius, both of whom have fallen in love with her, and activates the Anti-Magic Area spell. At the same time, Reyheim also activates Prison Field. The double barrier prevents Shion and the other fighters of Tempest from fighting as well as usual, and they end up at a disadvantage in their fights with three Otherworlders. Benimaru tries to contact Rimuru, who still hasn't returned, but he can't even get in touch with him due to the barriers.

Rimuru faced grave danger in his battle with Hinata, but he managed to escape in the nick of time by using a doppelganger to take the damage while he rushed back to Tempest. Upon arrival, he found the city in a tragic state, engulfed by blood and smoke. Benimaru tries to capture Myulan, the top suspect behind this disaster, but Youm and Grucius stand in his way. Swallowing his rage and grief, Rimuru realizes who their true enemy is and calmly questions Myulan instead of punishing her.

Rimuru learns about Clayman and the schemes of the Kingdom of Falmuth, but no matter how hard he wishes for it, his fallen comrades and their peaceful lifestyle will never return. He's just about to give up when Eren tells him of a fairy tale that might offer a glimmer of hope. The probability of success is about 3%... but if Rimuru can manage to become a Demon Lord by taking in the souls of 10,000 humans as nourishment, he just might have a chance at bringing his comrades back to life. As Falmuth's army of 20,000 men marches toward Tempest, Rimuru takes hold of that hope and decides to become a Demon Lord.

In order to come to terms with his own negligence and revive Shion, Gobta, and his other fallen comrades, Rimuru reveals that he used to be a human. But his allies think nothing of it and accept him regardless, as if it's the natural thing to do. Filled with gratitude for the warm, accepting environment he's in, Rimuru turns his focus to the army from Falmuth. He entrusts Benimaru and the others to destroy the devices that are generating the enemy's Prison Field while Rimuru himself sets out to attack the advancing army and use them as nourishment to become a Demon Lord.

A feast is thrown to celebrate the revival of Tempest's fallen and the return of Veldora. After enjoying a brief, peaceful respite, Rimuru tells everyone his future plans: he's going to get the word out to the whole world that he's a Demon Lord and declare war against Clayman. Then King Gazel of the Armed Nation of Dwargon, Fuze of the Kingdom of Blumund, and Eren's father, Archduke Elalude of the Sorcerous Dynasty of Thalion, all arrive in Tempest, and an important meeting to discuss the nation's future begins.

As prominent figures from several nations assemble in Tempest for a meeting, Laplace and his companions formulate a plan centered around Clayman. Then someone familiar joins the discussion...

Rimuru announces his plan to back Youm as the new king of Falmuth. After hearing this, Gazel tests Youm to see if he has what it takes to be a ruler, and Youm answers with full sincerity. Erald also takes a liking to Rimuru after hearing him say he just wants to create a world where everyone can prosper and smile. Thus Rimuru and his allies look ahead to the battle against their new enemy, Clayman.

A new force has suddenly risen to power in the Great Forest of Jura, and its leader, Clayman, proposes that the Demon Lords hold Walpurgis with the goal of sanctioning the slime who deceived him. Rimuru also begins to prepare for a battle with Clayman, but observing the enemy army's movements reveals that Clayman's target is not Tempest, but Eurazania. Clayman's army will arrive in Eurazania within two days. Can Rimuru's forces get there in time?