Six Ogres are all that remain of their clan after an attack by an Orc army. They say that the Orcs were traveling with a mysterious majin who wore plate mail and a clown mask. Rimuru proposes that the Ogres join his ranks, and they accept on the condition that they will get to take down the one leading the Orcs, so Rimuru gives all six of them names. Meanwhile, the Orcs begin to invade the marshlands surrounding Lake Siss that are inhabited by the Lizardmen. So the Lizardman chieftan gives his son, Gabiru, an order...

Rimuru's village has gained Benimaru and the other five Ogres as its newest allies. Soon after, the Lizardman Gabiru visits the village, having heard rumors about it. Gabiru has been visiting Goblin villages throughout the Great Forest of Jura to enlist their aid in fighting off the Orc army. When he faces Ranga with an attitude of provocation, Ranga tells him that if he can defeat Gobta, they'll hear what he has to say...

The Orc army, led by the Orc Lord, continues its conquest throughout the Great Forest of Jura. The crisis has spurred Treyni of the Dryads, the caretakers of the forest, to take action. She has had her eye on Rimuru after noticing his rise to prominence in the forest, and she wants Rimuru to defeat the Orc Lord.

Rimuru and his Goblin riders set out for the marshlands to finalize their alliance with the Lizardmen. On the way, they save a Lizardman who'd been attacked by the Orcs. The victim turns out to be Gabiru's younger sister, who reveals that Gabiru led an insurrection and left to fight the Orc Lord without waiting for the alliance. She also pleads with Rimuru to save Gabiru and the rest of her clan. Meanwhile, Gabiru faces the massive Orc army and is shocked when he witnesses the power of their unique skill, Starved.

The majin Gelmud makes his appearance at last. His true goal was to evolve the Orc Lord into the Orc Disaster, a Demon Lord, and have him reign over the Forest of Jura. Upon learning this, the Orc Lord takes action...

Post-war arrangements following the battle with the Orcs have begun. At the meeting, Rimuru reveals that he made a promise with the Orc Disaster, Geld, to take all of the Orcs' sins upon himself. When the Ogres and Lizardmen hear this...

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.