The Jura Tempest Federation, with Rimuru as its leader, has formed an alliance with the Armed Nation of Dwargon. Just as Rimuru thinks there are only peaceful days ahead, he senses an enormous mass of magical power heading straight for his location at an incredible speed. When the smoke from its collision with the ground clears, standing in its place is a humanoid figure. She is the Demon Mord Milim, the one and only Dragonoid and the one bearing the nickname "Destroyer." Ranga, Benimaru, and the others who face her are quickly overwhelmed by her power.

Its seal removed, the calamity-class monster Charybdis and its Medalodon minions advance on the capital city of Rimuru. The Jura Tempest Federation combines all its forces under Rimuru to intercept the attack. King Gazel even sends 100 of his Pegasus Knights to aid in the battle. But Charybdis's extra skills Magic Jamming and Ultraspeed Regeneration make it a formidable foe. So Rimuru unleashes his new skill, Gluttony...

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.

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.

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.

The battle for revenge has begun. Benimaru and his trusted companions carry out their respective missions, while Hakurou and his party face Shogo and Kyoya once again. And Rimuru's fury rains down upon the army from Falmuth.