The narrative begins with Satoru Mikami, a 37-year-old corporate worker in Tokyo, who is stabbed during a random attack. As he dies, he hears a mysterious voice and is reincarnated in a fantasy world as a slime, a low-level creature with unique abilities. Upon awakening, he discovers he possesses the skill "Predator," which allows him to absorb and mimic the abilities of anything he consumes. He also gains the ability to communicate telepathically. In this new world, he encounters Veldora, a powerful Storm Dragon sealed in a cave. The two form a bond, and Veldora names the slime "Rimuru Tempest" after a suggestion from Rimuru himself. In exchange, Rimuru promises to find a way to free Veldora from his seal.

After absorbing Veldora's essence to analyze the seal, Rimuru ventures into the world, gradually building a community of monsters and humans. He establishes the Jura Tempest Federation, a nation where different species coexist peacefully. Rimuru's leadership and diplomatic skills, combined with his growing powers, allow him to form alliances with neighboring nations and fend off threats. Key characters include Shion and Shuna, ogres who become loyal subordinates; Benimaru, a skilled warrior and leader of the ogres; and Milim Nava, a Demon Lord who becomes Rimuru's close ally. Rimuru also encounters other Demon Lords, such as Clayman and Carrion, who challenge his vision for the world.

The story explores Rimuru's journey from a solitary slime to a powerful leader, balancing political intrigue, battles, and the development of his nation. Major arcs include the founding of Tempest, the war against the Kingdom of Falmuth, and the Walpurgis Council, where Rimuru is officially recognized as a Demon Lord. Throughout the series, Rimuru's abilities evolve, and he gains new powers, such as "Great Sage," which later transforms into "Raphael," an advanced analytical skill. The narrative emphasizes themes of cooperation, growth, and the challenges of leadership in a world filled with diverse factions and conflicting interests.