John had recently just got his job. Before that, he was just doing his university work and watching One Punch Man in his spare time. He was walking from his office until he got attacked by a bunch of gangsters. They mugged him and stole all of his money. He had then realised they would murder him.
After some time, he woke up in a strange void. He called out, "Hello! Is there anyone here?" An extremely old man approached him. John was terrified. The old man said, "There is no need to be terrified. I will not harm you." John was puzzled and asked, "I want to know some things. Firstly, how did I get here? Secondly, what happened to me that caused me to come here? And thirdly, who are you?"
The old man answered," I am a heavenly being. You got here because you died after getting mugged by some gangsters. Since I feel pity for you, I will allow to reincarnate into any world you want, have the ability to choose your power and who you get to be reincarnated as." John replied,"Thank you so much, sir! I would like to be reincarnated as Saitama from One Punch Man and I would like to inherit his physical power."
"Well then, John. I shall transport you to the One Punch Man universe. Remember, your life there will be different from here. If you die, then you will not be allowed to reincarnate into that universe ever again. Goodbye, John."
And so John got transmigrated into the One Punch Man universe.