Mikami Satoru, a businessman, is stabbed by a criminal on the street and killed. When he regains consciousness in the darkness, he finds that he's been reincarnated as a slime! With nothing better to do, he spends his time devouring all the rare herbs and precious ores he comes across. As he does, he encounters the Storm Dragon Veldora, who was sealed in this cave 300 years ago by a hero's "Unlimited Imprisonment" skill. He's frightened at first, but as they talk, he becomes friends with the long-isolated Veldora.

The slime Mikami and the Storm Dragon Veldora decide to give each other names. Veldora gives the slime Mikami the name "Rimuru," and Rimuru comes up with a family name for both of them: "Tempest." Then Rimuru uses his Predator skill to take Veldora into his body and analyze the "Unlimited Imprisonment" seal in hopes of breaking it... but this action sends a shock through the nations surrounding the Great Forest of Jura. With Veldora gone, there's a chance that the larger nations could become bold and invade the smaller provinces. With no knowledge of this, Rimuru leaves the cave and meets some Goblins.

After leaving the cave, Rimuru encounters a village of Goblins under attack by a clan of Direwolves. Caught in the heat of the moment, Rimuru agrees to help the Goblins in their battle. That night, the Direwolves arrive to try to take over as the new rulers of the Great Forest of Jura!

Rimuru and his companions visit the Dwarven Kingdom, the Armed Nation of Dwargon, in search of some artisans to help the Goblins learn to supply their own food, shelter, and clothing. However, a skirmish just outside the city gate lands Rimuru and Gobta in a prison cell. Then there's a report of an Armorsaurus in the nearby mines!

Ogrza księżniczka, Shuna, a także towarzyszący jej Hakurou oraz Souei, pokonują nieumarłego króla, Adalmana, a także jego armię zombie, i ruszają na zamek Claymana. Co więcej, Palec Wskazujący przechodzi na ich stronę. W tym samym czasie rozpoczyna się Bankiet Królów Demonów, Walpurgis, gdzie Rimuru zasiada przy jednym stole z władcami świata, do którego został przyzwany...

Clayman, leżąc na deskach, w akcie desperacji prosi o moc, w wyniku czego dochodzi do jego przebudzenia. Rozpoczyna się prawdziwa walka! Rimuru postanawia rozprawić się z łotrem sam, by zyskać akceptację innych królów demonów. Jak zakończy się ten intensywny Bankiet Walpurgis?