Takemichi Hanagaki is a freelancer that's reached the absolute pits of despair in his life. He finds out that the only girlfriend he ever had, in middle school, Hinata Tachibana, had been killed by the ruthless Tokyo Manji Gang. The day after hearing about her death, he's standing on the station platform and ends up being pushed over onto the tracks by a herd of people. He closes his eyes thinking he's about to die, but when he opens his eyes back up, he somehow had gone back in time 12 years. Now that he's back living the best days of his life, Takemichi decides to get revenge on his life.

The seemingly ordinary and unimpressive Saitama has a rather unique hobby: being a hero. In order to pursue his childhood dream, he trained relentlessly for three years-and lost all of his hair in the process. Now, Saitama is incredibly powerful, so much so that no enemy is able to defeat him in battle. In fact, all it takes to defeat evildoers with just one punch has led to an unexpected problem-he is no longer able to enjoy the thrill of battling and has become quite bored.

For the agent known as