Off the Deep End

"It was at the beginning of summer 1732 however, that Captain Pines overheard the Tent of Telepathy's deck cadet rant about lost treasure in Mindscape Bay. The boy talked and talked and honestly, Pines had long since tuned him out – lost treasure, yeah right! But when he heard the word 'siren' being spoken, he immediately perked up. When pressed, the boy, a snot-nosed little brat in Stan's humble opinion, started to wax a grand tale about beautiful mermaids, gold beyond imagination and close calls with death. The Captain promptly told him to shut up. He walked out of the inn that night with a new expedition in mind and a smile on his face."

AKA the merman!AU nobody asked for, wherein Bill is a siren, everybody else is a pirate and Dipper is somehow (maybe) magical. (Can also be found on AO3)

Cover art by maplespyderart