George's parents got an invite to the Steadman Resort on Triple Tree Lake from Henry and his wife, the owners and friends of theirs. Since they can't go, they let the girls take their place. Nancy and her friends quickly find a gorgeous lake with dozens of things to do and tons of people.
Not long after arriving, they hear a scream from a neighboring cabin and rush over. Ann and Carrie, two sisters who work at the resort, came back to find their cabin trashed. Nothing is missing but they explain that it's just another weird thing to happen. They also give them the low down on people who work there like Rodney, the horse trainer who is a total hottie.
On a canoe trip, they meet Steve, yet another hot guy. He points out Horse Island to them and tells them that under no circumstances should any resort guest go out there. Legend says that a family once lived there, died in a mysterious way, and now haunt the island. Plus, hunters go out there to hunt illegally and once shot two guests. It gets its name from the wild horses who once lived there.
One of the guests on the canoe trip starts talking about how she wants to leave really badly. All this weird stuff keeps happening, but they keep ignoring it because their kids want to stay. Nancy talks to Henry, who admits that some suspicious things have happened and that he wants to keep everyone calm. Our hero also has a run in with a guest named Teresa who acts weird and doesn't want to talk to anyone.
The girls borrow a canoe and sneak over to the island. George brings along some guidebooks and starts pointing out all these birds and flowers. They find what looks like a brand new fence on a deserted island and the old mansion. After a quick investigation, all they find is a tower filled with bats and some minor proof that someone has been living there or at least staying there.
Sadly, a storm rolls in and when they head back to the canoe, it's gone. There's also some proof that someone dragged their canoe away, so they go back to the mansion and try to get some sleep. The next morning, Bess comes dangerously close to eating poisonous berries before George recognizes them from her book and stops her. Nancy uses her camera to take pictures of everything they see and later sees Teresa on the water. Though she looks right at the girl, Teresa takes off and leaves them there.
Bess is convinced that she saw a ghost the night before and shows them the exact spot where she saw it. Nancy finds a pair of sunglasses there, which a ghost probably wouldn't wear. The girls also wind up nearly trampled by a herd of wild horses, which is odd since horses supposedly no longer live on the island. Steve comes by with a group of guests on a canoe trip and helps them. He doesn't recognize the sunglasses but takes them and agrees to ask around and find the owner.
The next day, someone sets fire to their cabin. Henry rushes over with other staff members to put it out before it can spread. Nancy finds proof that someone deliberately set the fire but did it in a way that stopped it from moving beyond one room. The only thing destroyed in the fire was her camera with the photos from the island still on it. Gee, I wonder what Nancy would think about cell phone cameras!
On a horseback riding trip, their group comes across a dead body hanging from a tree. Nancy gets close enough to learn that it's actually just a dummy set up to look like a real guy. That pushes one more family to leave. Yet more people leave at dinner that night. Everyone who eats the blueberry pie becomes dangerously sick, including Bess. It turns out that someone put the poisonous berries in the pie. Though Nancy suspects the resort's chef Otis, he claims that someone stole a resort pan and made the poisoned pie.
While walking back to her cabin, someone hits Nancy from behind and pushes her into the resort swimming pool, almost drowning her. That person leaves behind a blue bandanna, exactly like the one Teresa wears. George and Nancy follow her into town and see her making a call on a pay phone. They confront her, she claims someone stole her bandanna, and she continues acting like a major bitch to both of them.
Nancy decides that the mystery must somehow be related to the island and goes back by herself. She sees Teresa following her through the woods and tries to ignore her until she hears Teresa scream. The other woman fell in a pit that someone set up as a trap. Even after Nancy saves her life, Teresa still acts like a witch. She finally confesses that she's a writer and working on a story but refuses to say anything else.
After following her back to the shore, Nancy sees Teresa have a conversation with Rodney, the horse trainer. A sudden gunshot rings out, and Nancy ducks to hide but instead finds that someone turned the fence into a corral to capture some of the wild horses. Someone knocks her out, and she wakes up in a canoe with Steve.
Steve confesses his whole plan to her because he assumes she'll die soon enough. He captured the wild horses with plans to sell the animals to others. He also set up the whole haunted island thing and did all the other weird stuff to try and make the resort go under. Steve assumed that he could then guy it back from Henry for a fraction of its value. After taking her back to the old mansion, he leaves her tied up while he deals with other stuff.
It's clear he doesn't know Nancy. She breaks an old clock and uses the glass to cut through her bindings before sneaking into the tower and hiding. She makes just enough noise to let Steve know her location. When he goes to the tower, she scares the bats enough that they freak out and attack, which gives her time to run downstairs and lock the tower door behind her. Teresa and Rodney then show up with the cops to save her.
It turns out that Steve brought Rodney in on his plan because he needed help with the horses. Rodney started feeling bad about catching the wild animals and contacted Teresa at her newspaper with the story. They secretly worked together to bring down Steve for weeks, but Nancy did it in mere days.
*Multiple people suspect Otis, the resort chef, because he's surly and complains a lot about not getting paid enough. Nancy thinks it's strange that he keeps bringing up how much he makes, but even Henry admits that people come back just for the food he makes and that he's seriously underpaid.
*The girls come back one day to find the cabin ransacked and completely flip out until they find a raccoon in their kitchen. Nancy is smart enough to trick the raccoon outside with crackers.
*The whole poisoned pie thing was kind of funny because George stopped Bess from eating the berries because they might kill her. Multiple guests then ate the pie laced with those berries and basically just had some mild discomfort. They didn't even need to go to the hospital!
*The sunglasses they found were Steve's, which Nancy figured out after way too long. He was the only one in the resort with prescription lenses.
*When they first get to the resort, the girls all change into skirts and tops for dinner. A rustic lodge in the middle of nowhere doesn't really seem like the type of place that would have a dress code for dinner.