inevitable undertale / one piece crossover (”underpiece”)
The land of Mt. Ebott is, as it turns out, just one isolated island on a vast sea. A sea that has entered the Pirate Era, ever since Gold Roger, “King of the Pirates”, declared he had hidden all his treasure… in One Piece.
Basically, imagine if Luffy was the 7th fallen human. Shortly after leaving Thriller Bark, the Straw Hats stop at an unusually developed island. They hear a legend that whoever climbs Mt. Ebott never comes back down; Luffy obviously climbs it, obviously falls down the hole.
Flowey greets him, offers him… “friendliness pellets”. (“Thanks, Flower-thing!” says Luffy, as he runs right into them.”) But wait! They bounce right off! (“How…?!” “Ah! That must be because I’m a rubber man!”) A few evil flower-faces and threats of violence later, Flowey is knocked out and left behind as Luffy enters the ruins…
…and runs into Toriel! He has a good exchange with this cow-goat-lady (although she’s too flustered to properly explain if she poops or not), eats some pie (as if he needs it to cool down), and soon is ready to get outta there. Toriel tries to stop him. He’s so foolish… if he goes out there, he’ll be killed, like the children were, by ASGORE… (“…So, that just means I have to kick this Asgore guy’s ass, right?” “No one has ever beaten him before! He is the strongest monster in the underground, with six human souls!”) but he ignores her, and punches right through the door. (“You sure are stupid, cow-goat-lady! After all… I’m strong!!”)
It doesn’t take long for him to encounter two hilarious skeletons. He finds out immediately they don’t poop (“COOL”), and no, they don’t even know what a devil fruit is, of course they haven’t eaten one. Anyway, having befriended the skellies, he goes to Waterfall, and has an ultimate shounen-style hero face-off with Undyne.
the Straw Hats realize that, of course, Luffy climbed Mt. Ebott. They pack it up and go after him. They meet Toriel, who, seeing they’re Luffy’s friends, tell them what happened and point them in the right direction. They leave, although Robin decides to stay behind and ask Toriel a few questions about the ruins and the fascinating history of the island. Monsters are, after all, able to live much longer than humans.
The remaining crew decide to split up before leaving Snowdin. Brook has met two fellow hilarious skeletons, and that’s just not an encounter you pass up when it happens. So he sticks around in Sans/Papyrus’s house. Sanji stays too, at the promise of visiting Grillby’s later. (Monster food? That’s got to be interesting.) Zoro’s already lost.
Luffy’s had a solid fight with Undyne, but as is his style, he’s bounced all over the place dodging her spears, eventually ends up in the Hotlands. Undyne collapses near the water cooler. Oh my god! The fish lady is dying! She’s a fish lady, she needs water! Luffy pours the whole cooler on her. Undyne backs off back to Waterfall. A weird fish.
Luffy walks on, until he sees a super cool laboratory in the middle of the Hotlands. (“Awesome!!”) He meets Alphys, who’s a lizard, so that’s cool, and she doesn’t poop, so that’s cool, although she’s a little on the annoying side to him. Until he finds out that she’s built a human-killing robot thirsty for blood. YES, THE LIZARD IS CONFIRMED FOR COOL. (But the robot is cooler.)
He gets the cell phone and bounces over all the puzzles, until he gets to the cooking show. See, now he has a jet-pack. Now he doesn’t have to bounce over puzzles, he can JET-PACK OVER THEM. And he does.
Nami and Franky make it to the lab. (Chopper and Usopp weren’t ready to stop lying on the floor and feeling like garbage.) Alphys meets them, definitely not expecting this, and not really sure how to react to Luffy already having cool friends he goes on adventures with. But it’s cool, really, the more the merrier. The only problem is they’re going to have to solve some puzzles, since nobody deactivated them. At first they go through them (Nami even manages to solve the tile puzzle), and Franky splits off to get some garbage for Alphys. (Robotics at work!)
But Nami senses something off, and confronts Alphys about it. Eventually, she learns about the True Lab, and the unfortunate experiments that haunt Alphys.
Robin and Toriel decided to head out after all. Robin’s been exploring around, checking out the ancient writings and whatnot, and Toriel heads for Asgore’s place. She has a fight to stop.
Zoro still doesn’t know how to get out of Temmie Village.
Mettaton’s show’s ratings reach an all time high, what with a rubber man facing the Underground’s greatest star, live. (Chopper and Usopp, watching with Blooky at their house, Lose It over this wild show. (“‘Shows’ are amazing!”) The skeletons are also watching with Undyne and their two pirate guests– Brook is entertained and Sanji is just… lost.)
(Alphys recovers Mettaton’s body, Franky is awed at her work, they nerd out, she gets a bit of a self esteem boost.)
In the end, Luffy finds Asgore, and he kicks his ass.
Kill him? What is he, stupid? Now he just needs to kick the barrier’s ass.
That doesn’t work so great, sure. But just in the nick of time his nakama (also weird but cool monster pals) arrive! And together—!
Together, they face Asriel Dreemurr, Hypergod of Death, or something along those lines. It’s an epic fight, to be sure. But they win. And when they win, Asriel goes back to being a flower. Flowey the flower.
In spite of everything, Luffy grins right at him.
Anyway that’s the 1k-word tldr on how Flowey becomes the Straw Hat house plant.
(He stays on the deck and yells at everyone that gets on.)
Shoutout to @hosekisama and @briochebread for yelling about this with me and coming up with an even portion of the ideas. I didn’t put them all in because I’m lazy and also said I’d finish writing this today. But I encourage everyone to add on more garbage (or change some existing garbage) because let’s be real this is the best undertale crossover ever.
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