Life gets pretty boring when you can beat the snot out of any villain with just one punch.

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!

An emergency summons gathers Class S heroes at headquarters...and Saitama tags along. There, they learn that the great seer Shibabawa left the following prophecy: "The Earth is in danger!" What in the world is going to happen?!