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!
When aliens invade Earth, a group of Class-S heroes finally finds a way to fight back and go on the offensive. Inside the enemy mother ship, Saitama fights Boros. Faced with the alien´s frightful power, he decides to get serious! What is the Earth´s fate?!