Rimuru Tempest's life in That Time I Got Reincarnated as a Slime begins with his unexpected and abrupt death in modern-day Japan. In his previous life, he was Satoru Mikami, an ordinary 37-year-old salaryman. One day, while trying to save his co-worker from a random knife attack, Satoru is fatally stabbed. During his final moments, he laments his mundane life and harbors a few dying wishes, including a desire for a more interesting life, and to be resistant to pain. These dying thoughts lead to his reincarnation in a different world, not as a human but as a slime—a creature often considered weak and insignificant in typical fantasy settings.

Upon awakening in this new world, Rimuru discovers that he possesses unique abilities, the most notable being "Predator," which allows him to consume other creatures and objects, acquiring their abilities and knowledge. This skill sets him apart from ordinary slimes, turning him into a powerful and versatile entity. Rimuru initially explores his surroundings in a cave and meets the Storm Dragon Veldora, who is sealed away due to a past conflict. The two form an immediate bond, with Rimuru naming Veldora and promising to find a way to free him. In return, Veldora bestows upon Rimuru a name, "Rimuru Tempest," granting him increased power. Rimuru then absorbs Veldora into his body, setting the stage for his journey.

As Rimuru ventures into the world, he encounters various beings and gradually begins to establish his own community. His first major interaction is with the goblin tribes, who plead for his help against a threat from nearby dire wolves. Rimuru defeats the wolves, asserting his dominance and later offering them a place in his new society. By naming the goblins and dire wolves, he grants them greater power and evolution, thereby earning their loyalty. This marks the foundation of what will become the Jura Tempest Federation—a nation where monsters of all kinds can coexist peacefully. As Rimuru continues to meet other races, such as dwarves, ogres, and lizardmen, he incorporates them into his growing nation, offering them protection, guidance, and the promise of a harmonious society.

Throughout his journey, Rimuru's influence expands as he deals with various threats and challenges. He confronts powerful adversaries, such as the orc disaster Geld, who leads a ravenous army of orcs. Rimuru defeats Geld and brings the remaining orcs into his fold, further increasing the diversity and strength of his nation. During this period, Rimuru forms alliances with key figures like the dwarf king Gazel Dwargo and the elf adventurer Shizu, whose tragic backstory and bond with a powerful fire spirit, Ifrit, reveal the darker aspects of the world's complex power dynamics.

Season 1 culminates with the conflict against the Charybdis, a colossal monster threatening the region, and Rimuru's subsequent battles with various factions, including demon lords who view his growing influence as a potential threat. Rimuru also faces personal trials, such as the emotional departure of Shizu, who entrusts Rimuru with fulfilling her wish to find and protect the children she once taught.

By the time Season 2 begins, Rimuru's nation has become a prosperous and diverse society, attracting the attention of neighboring human kingdoms and powerful entities. However, this newfound prosperity brings its own set of challenges. The human kingdom of Falmuth views the Jura Tempest Federation's rapid growth as a direct threat to their power and influence. They conspire to provoke a conflict, leading to an attack on Tempest that results in devastating casualties, including the loss of some of Rimuru’s closest allies, like Shion. This tragedy deeply affects Rimuru, who is usually calm and composed, pushing him to make a drastic decision.

In response to this attack, Rimuru seeks to evolve into a True Demon Lord to gain the power necessary to resurrect his fallen comrades. He orchestrates a massive battle against Falmuth's forces, using their souls to fuel his transformation. The successful evolution grants Rimuru immense power and allows him to bring Shion and the others back to life. This marks a turning point for Rimuru, as he fully embraces his role as a powerful and influential leader who must now navigate the complexities of power struggles, political intrigue, and the responsibilities of ruling a nation in a world filled with rival factions and unpredictable dangers.

Up to the end of Season 2, Rimuru's journey is one of growth and transformation. He starts as a simple slime with a desire for an interesting life and evolves into a powerful Demon Lord with a thriving nation under his command. His story is a mix of strategic battles, diplomatic maneuvers, and the pursuit of a society where all beings, regardless of race, can live together in peace. However, this journey also reveals the darker sides of power, forcing Rimuru to confront the harsh realities of his new world and the difficult choices that come with leadership.