The Ultimate Crossover
Chapter 1: After The Monster Association (One Punch Man)
"It's been a long day hasn't it Genos?" said the bald-headed, middle-aged dude that everyone in the world of anime knew as Saitama. A man with limitless strength.
"I agree, Master. In fact, I was a few floors above you while you were battling Orochi and witnessed you decimate the entire underground hideout. Punches of that scale should've shattered the tectonic plates beneath the ground, but you were- "rambled Genos on and on, but soon cut himself off, remembering his master's 20-word limit rule.
"Were you trying to flatter me Genos?" Saitama asked.
"Absolutely not, Master" Genos responded. "Just stating some facts, I read on a website, that also relate to hair growth- "
"AGAIN GENOS, NO ONE SAID ANYTHING ABOUT HAIR?!" Saitama screamed as steam blew out of his nostrils.
"Apologies, Master" said Genos, as he bowed so deeply his head touched the ground.
"If you want to give me facts, then could you tell me a way to quickly rebuild my apartment so I can catch up on the last few hours of sleep. It's actually really healthy for you Genos." Saitama queried.
"Yes Master." Genos replied, "The apartment, if I recall correctly, was here, but I do not think we can rebuild in one day, Master,"
"Damn it, now what?" Saitama said.
"How about Dr. Kuseno? He wouldn't mind us staying over for the night until we can find a new apartment, Master," Genos suggested.
"You sure the old geezer won't mind?" Saitama asked.
"Absolutely not, Master," Genos said. "In fact, he has been saying he would prefer company to his regular solitude,"
"Alright then, we'll stay at his place for a night and go house hunting tomorrow. Preferably in the ghost town area, because the places around there are pretty cheap and there aren't any neighbours to annoy us," Saitama explained.
"Agreed, Master," Genos said, and they both started walking towards the rising sun.
In an hour, they reached Dr. Kuseno's lab where they saw a short and shrivelled old man with a haircut that made his head look like a humongous mushroom. His laboratory was in a dense forest which is why Genos and Saitama looked a little tattered, as they had been running through the wilderness to avoid hostile wildlife.
As for the laboratory itself, the walls were mainly black, with shades of silver and blue lighting, with one beam of light in the middle and a few smaller lamp lights around, yet somehow the room was still incredibly dark. There were three large tables in the middle with machinery all over them, "Maybe upgrades for Genos" Saitama thought. All in all, the lab gave an eerie but relaxing feeling thanks to the blue and silver lighting.
Saitama had met the small old man once before, when he came to his apartment the night after he eradicated Elder Centipede. He had showed up wearing mechanised armour, but the best part was that he had brought premium meat with him, which Saitama used to cook up a hotpot.
"Ah, Genos, welcome back. It's been 84 hrs 59 mins and 59 sec since I last saw you." exclaimed Dr. Kuseno, in a voice that made him sound ten times wiser than he was, if that was even possible.
He was, to put it mildly, the smartest man that Saitama knew. He repaired, and revived, Genos multiple times when Class S was dealing with the Monster Association's first attack on humanity and multiple times before that too. He also gave Saitama a handmade electric stove which worked more consistently than literally any other electronic that Saitama owned, including his TV that Genos gifted him on his birthday (He wasn't the richest guy in town), and somehow managed to survive Orochi's giant beam attack, so Saitama put it in his bag of collectibles that he'd been carrying around consisting of stuff that remained intact after having Saitama's apartment fall on top of it. So far, it consisted of one pair of socks, three pairs of underwear, two shirts, the stove, his one pair of pyjamas that he wore to bed and the bag itself.
"You too, Mr. Saitama. How was the premium meat I sent over yesterday?" the doctor asked.
"Wait!? You sent some over yesterday?" Saitama exclaimed. "I knew I shoulda checked the mail,"
"Would you like one my robots to fetch it for you?" Dr. Kuseno suggested.
Saitama didn't want to seem greedy, but he hadn't had anything to eat since he shared that Japanese Spined Lobster with the Class S Rank 3 hero Silver Fang at his dojo, day before yesterday.
"My robots would not mind" Dr. Kuseno said. On that note, a 3-foot-tall robot walked past the doctor, out of the main entrance and took to the skies.
"He should be back in about ten minutes. Five maybe, since that's the faster new model." Dr. Kuseno predicted.
"So, what brings you to my laboratory?"
"Master Saitama's house was obliterated in his battle against the Monster Association and we would like a place to stay tonight or at least until we can find a new residence for Master and I," Genos explained.
"Oh, well in that case, stay by all means until you have found a suitable residence for yourselves. In fact, I was meaning to call you over Genos for I have an upgrade I would like to test, and it was ready sooner than I scheduled it to be." Dr. Kuseno said.
"Gladly, Doctor." Genos replied.
"As for your living quarters, they're one floor below and three doors to your left." Dr. Kuseno explained.
"Thanks" said Saitama.
He started to unpack once he entered the living space which, fortunately, was bigger than two-thirds of Saitama's apartment. It had the same wall colour as the lab itself, but the lighting was normal. It had a massive bedroom on the left, "King-Size, most likely" Saitama estimated, and an enormous bathroom to the right with a lot of storage options in the middle. Granted, Saitama didn't even half the stuff needed to fill up all the shelves and racks. Regardless, it was a nice feeling.
He started taking his red gloves and boots and his yellow jumpsuit and cape off to be dry-cleaned. Saitama didn't want to sound like an egotistical and spoiled brat, but he was more than sure that the doctor had some sort of dry-cleaning method if he could make a stove that was pretty much bulletproof.
After Saitama changed into his pyjamas, he heard a loud explosion behind the wall his bed was placed next to.
"Seriously?" Saitama said, exasperated, to himself. "I just changed out of my hero outfit,"
He had to investigate, because he couldn't enjoy his sleep on his comfy bed with loud noises right outside his door. He went out the way he came, ready to get his only clean clothing dirtied with monster insides and ready to fight, when he realised it was actually Genos trying out his new upgrade. It looked like a mini-rocket launcher on his shoulders and some backwards facing lasers.
"I cannot thank you enough Doctor. You have pulled through for me again," Genos thanked the Doctor.
"Thank goodness it wasn't a monster," Saitama said, yawning, as he came out if the bushes.
"My apologies for disrupting your sound sleep," Genos apologised, bowing deeply.
"No problem. Anyways, I just came to say good night" Saitama said as he walked back into his room and fell asleep the second, he hit the bed.
Five hours later
Saitama jolted awake as he felt the ground trembling beneath him. Normally, his absent-mindedness would register it as an earthquake, but he could also hear thumping. Either heart beats or footsteps. With lightning speed, literally, he threw on his hero costume and walked out of the door, still sleepy but ready to fight. He looked left and right and couldn't see anything until he looked upwards.
He saw what looked to be a huge pair of violet eyes. The towering being looked to be a man with massive club in his hand and was laughing his head off. A few seconds later, Genos and the Doctor, in an armoured mech suit equipped with weapons, came out of the front door to examine the source of the ground trembling and looked upwards to see the giant monster.
"HAHAHAHAHAHAHAHAHAHAHAHAH" The monster was laughing his lungs out until Saitama ran out of patience.
"Shut up!" Saitama said louder than usual.
"Puny human. I am GRAAAH- "The monster's sentence was cut off as Saitama's almighty fist made lethal contact with his cranium. The force of such a simple motion was so strong, that the monster's head went a complete 360 degrees on his neck and fell off. Not to mention, the shockwave that was procured had enough energy to demolish the entire mountain they were standing next to. To put it in perspective, it takes 15 kilotons to take over 600 ft of the top of a mountain and wind speeds of around 180-200 kph to completely uproot half a forest. However, this punch annihilated the entire mountain and decimated the entire east section of the forest and, most likely, blew some fast winds into City M on a day which forecasted not to be windy.
"I hate it when monsters think that they can do whatever they want and disrupt a man's peaceful sleep in his king-size bed," Saitama exclaimed as he landed.
"It's alright, this looks just about over anyways Dr. Kuseno," Genos said as a small smile graced his face, showing his unwavering faith in his master's power. He turned around to see Dr. Kuseno frozen in awe at Saitama's overwhelming power.
"How did you attain this power?" Dr. Kuseno asked but caught himself. "My apologies. I'll ask questions in the morning,"
It seemed to be over once the giant corpse fell down, but then it started to glow purple. The whole thing then shrunk to the size of an average person and eventually shaped itself into a misty oval. This oval seemed to have an incredibly powerful gravitational pull and started pulling everything in, including the soil, anthills, roots, and trees. Saitama was fine as usual, but he saw Genos getting pulled in at an alarming speed, so he leapt forward to grab him and pull him out. The portal started to close, and Genos was already stomach deep in it. He managed to fire a grappling hook with his free hand to a tree and activate some boosters to propel himself out, but the tree got uprooted and the gravity was too strong. The gate then started to close, and Genos was already all the way through with the exclusion of his fingers that were still holding on to Saitama's hand. The gate quickly shrunk enough to just barely fit their hands through so Saitama decided to make it wider. He shoved his other free hand into the corner of the portal and started pushing it wide open. No one, human or monster or anything else in between, could've pulled of such a feat. His hand was pushing it open further until it was wide open for Genos to get through.
He was almost out until the tree that had been uprooted earlier collided into both of them. It broke in contact with Saitama but hit Genos hard and he lost his grip on his master's hand. He went all the way through, and the gate started closing once again but faster this time. Not as fast as Saitama though. He, once again, managed to hold the gate open but reaching Genos was out of the question. So, Saitama decided to jump through and reach Genos or at least go with him to wherever the portal's taking them. This, ladies, and gentlemen, is where the fun begins.