Rimuru defeats Clayman at Walpurgis Banquet, becomes a Demon Lord joining Octagram. Diablo anxiously proposes a contract with Veldora to learn about Rimuru's adventures. They discuss the banquet.
Satoru, a corporate employee, finds himself reborn as Rimuru, a slime in a parallel universe, after a random killing. He strives to establish a welcoming world for every race.
Devastated over Clayman and Roy's demise, Yuuki and Luminus grapple with sorrow. Now viewing Rimuru as a formidable Demon Lord, they contemplate their next steps against him.
As Rimuru territory expands to cover the entire Jura Forest, it was expected that Rimuru would receive greetings from various races. Rimuru comes up with the idea of holding a Founder Festival for diplomacy and acquiring new residents.
There are fears of human casualties caused by monsters ahead of the "Founder's Festival." Meanwhile, thanks to Vesta and Kaijin's inventions, there is hope for ensuring the safety of the highways. On the other hand, Rimuru gives Geld a new mission.