Oh God. Just when you thought things couldn't get crazier, Nancy's in a fucking plane crash.
So: Bess originally stumbles onto the moss-covered mansion in question while the girls take a detour off the main road somewhere, and hears a scream, and runs away. Nancy and George are intrigued, but George falls into an old fish pond and then loses her watch. A red-bearded man comes out of the house and orders them to GTFO. Nancy doesn't want to, but he has a damn gun.
The girls are on their way to a town called Ashley, to meet up with Carson Drew for his birthday. Nancy's present to him is an oil portrait of... Nancy. (Granted, Carson loves the gift... I mean, it seems odd, but he is her dad and he loves her, so I guess it makes sense.) The painting was damaged in transit, but Nancy and the owner of the boarding house where they're staying repair it. And the (black) cook makes a birthday cake for Carson. It's a big ol' party.
Carson is trying to track down June Campbell because she inherited a bunch of money from her dead mom. Nancy decides to help. They meet Ms. Labelle, who knew June, and Penelope, one of her old friends, who can help identify June if she's ever found - although Mrs. Labelle's fucking ceiling falls while the girls are visiting and Nancy's ass is knocked out.
Also NED COMES TO VISIT YAY, and on the last day of his visit Nancy and Ned go to "look at some gardens," which I like to think actually means "went parking and made out." And he tells her to come home soon and sends her a postcard after he's gone home.
The girls make several trips out to the scary house, mostly to find George's missing watch (which they do find). A fisherman warns them off, and they hear that a man was shot, taken to the hospital, and died. A swarthy jackass named Ramo claims it was his brother and that the girls know something and then proceeds to be shady af for the rest of the book. (The girls think he's bad and don't feel sympathy for his dead brother, especially once they find out he most likely didn't even HAVE a brother.)
A woman claiming to be June shows up, and Carson isn't sure about her but gives her the $52,000 anyway. (That would be about $846,000 now, so hell yes.) Of course she's a fucking imposter. She's a fortune teller's daughter and Ramo's niece.
Nancy and Carson get a lead about June, and decide to take a plane, but fog comes up and THE FUCKING PLANE CRASHES AND CARSON ISN'T BUCKLED IN AND A TREE FALLS ON NANCY WHEN SHE'S TRYING TO HELP THE PILOT. And then she wakes up and SHE'S IN THE MOSS-COVERED MANSION. She's so worried about her dad, though, that she leaves immediately.
So: the guy at the mansion is a famous artist. June arrives to pose for some artwork, WITH FUCKING WILD ANIMALS. Adjective, not verb. They catch the thieves and recover the money, and June gives money to poor Mrs. Labelle, who is broke and has heart attacks during the book and is broke. Nancy, along with surviving a FUCKING PLANE CRASH, is thrown out of Ramo's car when she tries to stop him from fleeing, rescues an escaped carnival monkey, and serves as nursemaid to Mrs. Labelle. She's totally the best.