It is just another ordinary day for Satoru Mikami. The sun is shining brightly on the streets of Tokyo, and he is in the midst of a discussion with his colleagues when suddenly he gets stabbed by a passing robber. The average 37-year-old corporate worker is now at death's door, and he has only one regret—dying a virgin. Fading out, he is startled by a mysterious AI-like voice reciting commands.

Satoru abruptly wakes up in the middle of a strange cave, but something odd has happened: he is now a goop of slime! Thanks to his new body, he has acquired the skill to absorb anything and obtain its appearance and abilities. While testing out his newfound powers and exploring the cave, he stumbles upon a massive dragon named Veldora, who has been sealed away for the past three hundred years. After quickly befriending the beast, Satoru decides to help him escape; in exchange, Satoru is bestowed a new name: "Rimuru Tempest."

Wishing to avoid a life like his boring and mundane past, Rimuru is about to set out on a grand quest that will change his own destiny and the fate of his new world.