Saitama vs. Overgrown Rover and Nyan

Saitama vs. Overgrown Rover and Nyan was the brief fight between the B-Class hero Saitama and the Monster Association executives Overgrown Rover and Nyan.

Prelude
While continuing his patrol through the Monster Association headquarters, Saitama comes across Overgrown Rover. He contemplates what could have allowed a dog to grow so large.

Battle
Rover growls angrily at Saitama, then fires an energy beam at him. Saitama jumps over the beam, calling Rover a "bad dog" and remarking at his lack of training, before jumping and launching a punch, which sends Rover flying.

Saitama's punch deals massive collateral damage, destroying a very large area of the Monster Association base. The shockwaves are large enough to be felt across the entire base, and are noticed by hero and monster alike. Sekingar wonders if the act is a sign of an unexpected party joining the conflict, while Genos realizes it could only be the work of his master.

Nyan, having witnessed the encounter, is shocked upon realizing Saitama's abnormal strength. He takes advantage of Saitama's unawareness and launches a sneak attack, scratching the back of Saitama's head several times with his Super Feline Retribution. However, Saitama is completely unfazed, turning around and remarking at the absurdity of encountering a monster dog and cat at the same time. A terrified Nyan flees the scene, deciding to leave Saitama to the other monsters.

Aftermath
Gyoro Gyoro contemplates the event, believing Rover's defeat to be an impossibility, but is cut short by the timely arrival of Tatsumaki.

Trivia

 * Nyan's interference is manga exclusive, although the majority of the battle with Rover is identical to the webcomic.