Saitama vs. Subterranean King

Saitama vs. Subterranean King was the fight between the amateur hero Saitama and the Subterranean King.

Prelude
Saitama is sleeping in his apartment. Suddenly, a giant hand breaks through the wall and grabs his head. He is thrown out of his apartment by the mysterious being, only to find his house destroyed. The monster then digs up from the ground and punches Saitama. The hero blocks the attack with his elbow, but is still launched into a nearby building and begins to bleed. Saitama looks up and determines that this foe is indeed very tough. The bald hero asks his opponent who he is, and the monster claims that he is the true earthling, with his population known as the Subterranean People. Several more Subterraneans come up from the ground, and one of them states that 70% of the Earth dwellers have already been killed, hoping that Saitama can understand this is a battle survivor. The Subterranean that attacked Saitama shows his surprise for finding a surface dweller that survives their punches. Saitama shares the same sentiment, stating that it's been too long since he's fought such worthy opponents, derisively calling them "molemen". This enrages the first Subterranean, and he attempts to punch Saitama, which Saitama dodges. Saitama yells and throws a serious punch that lands directly on the Subterranean's stomach, spewing its guts everywhere. The other Subterraneans are shocked that Saitama could kill one of them, and one of them clasps his hands together to kill Saitama. The hero dodges the attack and jumps on one of the Subterraneans to escape into the street. Three Subterraneans jump to attack him, and Saitama proceeds to expertly avoid their attacks, but then, all three Subterraneans join their attacks to strike the hero downward. Saitama holds his elbows up to defend himself, but the force of the attack is potent enough to sink the road down. Saitama's eyes flash with passion, and the hero sweeps the Subterraneans' feet from under them. He follows up with a loud shout and punches all three of them, killing the monsters instantly. Several more Subterraneans jump up to kill Saitama, but the hero is able to expertly take care of the Subterraneans using amazing and efficient moves. But out of nowhere, a larger Subterranean shows up and punches Saitama. The hero is unable to defend himself, and grunts as the force of the punch sends him flying. Much of the city is destroyed, and many more Subterraneans show up. They all express amazement at the unbelievable power Saitama possessed, but are shocked when they hear Saitama's voice. Saitama declares himself as a hero for fun, and proudly claims that the surface is guarded by him. The Subterraneans all rush Saitama, but the hero pushes them all away and punches one of the larger Subterraneans. As he's holding them back and being outnumbered, Saitama notes the feeling in his heart and the rushing sensation. While he continues to fight, he becomes elated, realizing how long it had been since he felt the exhilaration of a real fight. A longer fight ensues, and the scene changes: it has become cloudier, and a light drizzle befalls the city. Standing on top of a pile of corpses, Saitama is exhausted, breathing heavily and taking pleasure in the light rainfall drizzling on his body. The hero notes that all the memories of exhilarating battles are coming back to him. Then, erupting from the ground is the leader of the Subterraneans, who mockingly says Saitama has been taking good care of his children. Saitama smiles at the challenge issued by Subterranean King and runs up, jumping to fight the Subterranean King while realizing this was the feeling he was looking for.

Unfortunately for Saitama, this all turns out to be just a dream, and his alarm clock interrupts the battle before it could begin. Saitama smashes his alarm clock, mildly disappointed by this, but outside his apartment, he hears the sound of the true Subterraneans trying to claim the surface as their own.

Battle
Saitama immediately puts his hero suit on, and while the Subterranean King introduces himself and declares his superiority, Saitama kicks the Subterranean King's head, killing him with ease. The other Subterraneans are completely shocked by this, and Saitama yells with pleasure, telling them to bring it on. Alas, the Subterraneans flee, putting a white flag of surrender and returning to their headquarters, leaving Saitama surprised and disappointed to know he wouldn't be having an exhilarating battle.

Aftermath
The Subterraneans surrender and are significantly crippled by the death of their leader, while Saitama laments about his strength.

This might be just another throwaway gag created by ONE but this "battle" might suggest that Saitama may have some form of Precognition (similar to how Beerus dreamt about the Super Sayian God in Dragon Ball Super)