Tio, a young knight, had been on a journey to save the world with a female hero. However, one day he was expelled from the party due to his lack of skill. That night, Tio has a dream. He dreams of a Demon King who used black magic to defeat the demon god who plunged humanity into a crisis hundreds of years ago. Tio remembers that the Demon King was his past life. And that in the present day, the black magician, who is considered a bottom-tier profession, can become the strongest if he uses a certain method known only to him. Remembering his past life, Tio changes classes from a knight to a black magician and becomes known as the 'strongest'.