It's a movie about a young man named Allen. One day, Allen was rescued by sirens in an almost naked state after an accident at sea at a strange time. After the rescue, Allen discovers the mermaid and her new young man who start dating. One day, Allen accidentally discovers that he made a strange-looking Madison, where as she gets wet she turns into a fish tail.
It's a rambunctious comedy, coming from director Ron Howard who, with his previous, wacky Night Shift, unarguably shows that beneath that Opie Taylor/Richie Cunningham persona, there's a wild and crazy spirit.