Roland is an assassin who, one day, breaks from his team and single-handedly "kills" the demon king. After returning to the kingdom, the king graciously promises Roland whatever he wants for his heroics. Conflicted in his desires—he hasn't thought of anything outside of killing for years—he remembers that one of his teammates told him what she plans to do after evil is defeated:

"Live an ordinary life."

And so Roland, with his ability to become unnoticeable and the disguised demon king/queen Laila at his side, decides to live a slow life.

(Source: MangaDex)