Satoru Mikami, a 37-year-old office worker, leads an ordinary life in Tokyo, satisfied with his routine despite never having had a girlfriend. During a casual chat with a coworker, he is suddenly stabbed by a random attacker on the street. As he loses consciousness, a strange voice speaks in his mind, reciting a series of commands that the dying man cannot understand.
When Satoru wakes up, he finds himself reborn as a blob of slime in an unknown world. Along with this new form, he gains unique abilities—most importantly, the power to consume anything and copy its appearance and skills. He soon encounters the sealed Catastrophe-level monster known as the "Storm Dragon" Veldora, who has been trapped for 300 years after destroying a town. Feeling sympathy for the dragon, Satoru becomes friends with him and vows to help break the seal. In exchange, Veldora gives him the name Rimuru Tempest, granting him divine protection.
Freed from the tedium of his former life, Rimuru sets off on a new journey with a clear purpose. As he adapts to his new body, his slimy adventures begin to affect the world around him, gradually shaping his fate.