the lives of four detectives in their twenties are changed forever when after an experiment gone awry, three of them rapidly age overnight and find themselves in the bodies of old men in their seventies (only one, park jung-woo, emerges unscathed and stays young). as his suddenly much older colleagues comically struggle to deal with their new and unusual predicament, jung-woo shares in their hardships, joining them in their mission to find a way to reclaim their lost youth. meanwhile, the team also continues to solve crimes, while the "grandpas" work at a delivery service as their cover.