After a particularly rough encounter with Crampelter, resulting in a black eye for Stan and very hurtful words for himself, 8-year-old Stanford Pines has trouble sleeping. Outside his window he catches a glimpse of a shooting star and wishes that there was someone out there just like him; someone who understood what it was like to be an outsider. He falls asleep a short time later, unaware of the interdimensional presence that has just touched down in Glass Shard Beach.
The next morning, while combing the beach, he and Stanley make a remarkable discovery: a cave only accessible during low tide! And inside that cave is the rotting, ruined remains of an old ship, just barely big enough for two. As the boys examine it, they find something lurking within the bowels of the ship.
It looks like some kind of alien, squishy and triangular with one giant eye in the center of its face and a pair of oversized sneakers. When it speaks, it sounds like colors. Stan and Ford understandably flip out at the sight of a genuine, bonafide alien. Ford wants to take it and present it before the world's top scientists, but when he tries to grab it, the thing squirms away and scampers deeper into the cave. High tide is coming and the boys have no choice but to retreat and come back the next day. Maybe the alien will still be there.
Sure enough, the alien is and it once again evades capture. After several days and even more failed attempts, the boys decide to set a trap. They construct a crude box trap, using leftover spaghetti as bait, and successfully capture the alien! However, upon closer inspection, Ford and Stan begin to have a change of heart. The thing looks at them with a big, watery eye and they can't bring themselves to expose it, so they free it from the trap. Instead of fleeing, however, it regards them curiously. Stan and Ford attempt to communicate with it, first by telling it their names.
"Sta...ferd...Sta...lee..." the alien struggles to say.
Ford and Stan are overjoyed! They've just made successful contact with an alien! They ask it what it's name is and the alien doesn't seem to understand. That's when Ford pats his own chest and says, "Stanford. Stanford. Stanford." before doing the same with Stanley.
The alien's eye brightens and it excitedly stomps its little feet as it says, "Bill! Bill!"
"Guess its name is Bill," Stan says with a shrug.
They may be friends now, but they still need to figure out what to do with Bill. Stan suggests sneaking Bill home in a backpack and letting it sleep under their bed, but Ford shoots that idea down. Instead, they drag the ruined boat out of the cave, christen it the "Stan O' War" and get started on making the place liveable for their new alien buddy.
Basically this is an AU where child Bill accidentally ends up in Glass Shard Beach and grows up alongside Stan and Ford while living in the Stan O' War. (He gets a human form that he uses to attend school and move about the town, but otherwise stays as a triangle when alone with his pals). Of course he and Ford fall in love, so cue adorable puppy love blossoming into awkward teen romance while Stan watches his nerdy brother fall all over himself trying to successfully date their childhood friend/weird alien thing.