Life gets pretty boring when you can defeat any opponent with a single punch. Saitama, an average-looking guy with an unimpressive physique and a bald head, faces a unique problem—he's too strong. Finding a worthy challenger seems impossible, leaving him feeling empty and unfulfilled.

During a martial arts tournament, the formidable Suiryu of the Dark Body Art displays his remarkable strength, catching everyone's attention. Meanwhile, outside the venue, an onslaught of monsters is pushing the heroes to their limits, including Genos. Inside the stadium, Saitama, oblivious to the chaos, is set to face Bakuzan in his match.