Satoru Mikami, a 37-year-old office worker, is fatally stabbed during a random street attack. As he dies with regrets about his unfulfilled life, a strange voice echoes in his fading consciousness.

He awakens in a dark cave, reborn as a slime with the power to absorb anything and gain its abilities. While exploring, he meets Veldora, an ancient dragon trapped by a seal for three centuries. The two become friends, and Satoru promises to free him. In return, the dragon gives him a new name: Rimuru Tempest.

Ready to leave his mundane past behind, Rimuru sets off on an adventure that will transform both his destiny and his new world.