Saitama aka (One Punch Man) is a hero just for fun. In the beginning, Saitama was just a normal office worker who was having trouble actually getting a job. However was confronted by a big lobster monster and got into a conflict because the monster wanted to harm a child, even though he knew that the monster would have the upper hand in this one he was determined to win and did. After that he wanted to become the hero he dreamed to be since a kid. He would be a hero that would defeat his opponent in just one punch.
In the story Saitama trained for three years. He would do 100 push ups, 100 sit ups, 100 squats and 10 kilometers every day for those three years. And after those three years he obtained super human strength where he did defeat his opponents with just one punch. However after obtaining this power he became numb to fear and excitement during a fight. So in lame mans terms he would be bored to any villain that crosses his path. Or in his terms "there is no clash of spirits."
After making a cyborg friend named Genos, Saitama and Genos joined the hero association. The thing is Saitama didn't even know that this association existed. So in order to become official heroes Genos and Saitama had to take two tests, a physical test and a writing test. Genos scored perfectly on both tests, and Saitama scored perfectly on the physical test, however he just barley passed the writing test. So Genos came out to be ranked a class S hero and Saitama was a class C hero
Even though Saitama was ranked in a lower the hero association he would stop crimes and save the day raising his ranking up to rank C no. 1. He would be recognized but with a negative view because of his incredible destructive strength. and in the end he didn't care what the other people thought of him.
Soon a great threat would come to Earth. The hero association was the first to act leaving all the rank S heroes to the last hope for salvation. However they really didn't stand a chance against Lord Boros and his army, so Saitama soon shows up and challenges the power house and defeats him with not even using his full strength even though Boros was the strongest foe he had faced.
The battle didn't end in just one punch. It got quite out of hand to the point to where Saitama got launched to the moon. He landed unharmed and just jumped back to Earth. He ended with his "Serious Punch", leaving split in the atmosphere and saving the Earth.