Volume 15

"Those Who Move In The Dark" (暗躍する者たち, An'yaku suru Mono-tachi) is the 15th volume of the One-Punch Man manga series.

Summary
Nothing about Saitama passes the eyeball test when it comes to superheroes, from his lifeless expression to his bald head to his unimpressive physique. However, this average-looking guy has a not-so-average problem - he just can't seem to find an opponent strong enough to take on! Despite just finishing a martial arts tournament and immediately dealing with the Monster Association invasion. Saitama still proves to be so incredibly strong that he feels empty inside. King lectures him on how to live like a hero, but it doesn't help. Meanwhile, servants of the Monster Association close in on Speed-o'-Sound Sonic!