This is a possible concise summary:
One-Punch Man is an anime about Saitama, a bald hero who can defeat any enemy with one punch. He is bored of his life and seeks a worthy opponent. He meets Jenos, a cyborg who becomes his disciple, and joins the Hero Association to become an official hero. He faces various monsters and villains, such as the Sea King and the alien lord Boros, but none of them can challenge him. He is often unrecognized or misunderstood by the public and other heroes, but he still saves the world with his strength. The anime ends with a major disaster caused by the alien invasion, which Saitama stops with one serious punch.
Here are some key facts extracted from the text:
1. One-Punch-Man tells the story of a man named Saitama who is a 25 year old unemployed man who becomes a hero after training hard and gaining extraordinary strength.
2. Saitama can defeat any enemy with just one punch, but he feels bored and empty because he cannot find a worthy opponent.
3. Saitama meets a cyborg named Zenoss who becomes his student and friend. Zenoss admires Saitama's strength and wants to learn from him.
4. Saitama registers with the Heroes Association, an organization that ranks and assigns heroes to fight against various threats. He starts as a class C hero, the lowest rank, and later moves up to class B.
5. Saitama faces many enemies and challenges, such as the House of Evolution, the Sea King, the Heaven Gang, and the alien invasion led by Boros, the ruler of the universe.
6. Saitama defeats all of his enemies with ease, but he is not recognized or appreciated by the public or the other heroes. He is often mistaken for a criminal or a fraud because of his appearance and attitude.
7. Saitama still hopes to find someone who can make him feel the thrill of battle and restore his emotions. He also tries to be a good hero and protect other people with his strength.