"Rimuru Tempest" is the new name of a slime in a fantasy world taken by a former 37-year-old human Satou Mikami after he is killed by a passing robber. Broken free from ordinary, stale past life, his fresh adventure in a fantasy world as a slime monster with unique abilities begins.

Stories of old tell of four otherworldly heroes—wielding the sword, spear, bow, and shield—who defended the land from wave after wave of calamity. With the fate of the world in balance, the kingdom of Melromarc summons these legendary figures; in modern-day Japan, the call is answered, and the unwitting heroes are transported to this fantasy universe. Thrust into Melromarc and given the title of "Shield Hero," otaku Naofumi Iwatani is labeled the weakest due to his lack of offensive capability and apparent inexperience. When the heroes part ways to start their journeys, he only has one willing companion: the beautiful princess Malty Melromarc. However, she soon betrays him, steals all his money, and accuses him of taking advantage of her. For his alleged crimes, Naofumi is branded a criminal and made outcast of society. With hatred filling his heart, he sets out alone, vowing vengeance against those who wronged him.