Nelson Stone, the curator of the brand new Clinton Park Museum, calls Nancy and asks for her help solving a mystery. Though she isn't happy at the idea of skipping tennis, George agrees to go with her to the museum, and Bess puts off yet another shopping trip to go too. Clinton Park is a town not far away from River Heights. Amanda Lane, a super rich lady in town, donated her family home to the city to build a museum, and the city hired Stone to run it.
Nancy meets Su-Lin, a college student working as an associate curator, and her grandfather, who is the gardener. Stone takes her into the exhibit hall to show her the Golden Horse, the museum's prized possession. He recently spent one million dollars on it, which the museum board didn't like. Hillary Lane, the great-niece of Amanda Lane, is unhappy too because she lost the sculpture, which is made of real gold and jewels, to him at auction.
When they go back to his office to talk, he tells her that he got a threatening letter in the mail and that someone wants him dead. He then suddenly remembers that he didn't lock the Golden Horse back up, rushes to the exhibit hall, and finds that someone stole it. The cops come and launch a full investigation, but Nancy decides to check things out on her own.
She discovers a gold earring left behind and sees red purse in an office. Su-Lin tells her that the office belongs to Margaret Parker, Stone's assistant but that she's off for the day. When she goes back, the purse is gone. Hillary Lane then makes an appearance and whines about how the board never should have hired Stone. It turns out that she desperately wanted the curator job for herself, which makes sense because she's also an art history advanced degree holder with years of experience and a published author in the field.
Nancy winds up going to see Hillary under the guise of wanting to buy tickets for a big dog show she holds. Hillary is a little hesitant to let her in but quickly warms up to her because she knows Carson, Nancy's dad. She even takes her in to show Nancy her private collection of antiques, which includes a second Golden Horse. Hillary explains that it's part of a matched set, which is why she wanted the one stolen from the museum.
The girls wander across a car accident with Stone behind the wheel. He tells them that his brakes failed and it's proof that someone wants him dead. Nancy tracks down the mechanic working on the car and finds Margaret Parker actually inside the car. She claims that she lost an earring in her boss's car that morning, then gets flustered when Nancy asks her about the theft, and she claims that she wasn't actually at the museum that day at all. The mechanic finds proof that someone cut the brake lines with what looks like a hacksaw.
Nancy and the gang go back to see Stone and arrive just as he gets home. He finds a box of fancy chocolates on his porch, and despite clear hints from Bess, he acts really stingy and puts them away. Stone calls Nancy the next day to tell her that his neighbor's dog got in his trash, found the chocolates he conveniently threw away, and nearly died. Though she points out that eating an entire box of chocolates would make any dog sick, he's convinced someone tried to poison him.
Su-Lin then calls to tell Nancy that Stone fired her grandfather. She heads over to their house and learns that the man fired him after claiming that a hacksaw went missing, which he blamed on him. They tell her all about how he screams at workers all the time and acts like a pompous jerk. Nancy goes back to the museum to search for the hacksaw and actually finds it, but someone pushes her down, steals the saw, and then locks her in the tool shed. Su-Lin gives her an elegant and fancy necklace that is an antique from her family. She tells Nancy the necklace will keep her safe as long as she never opens the locket.
At the dog show, the girls meet Justin Todd and his wife, Fiona. Fiona falls in love with the necklace and asks if her husband can see it because he makes replicas of antiques. He flips out on her and claims that he would never do something like that. When she keeps talking about a job he just worked on in his jewelry shop, he pretty much grabs her by the arm and drags her away. She later overhears his wife talking about some big replica he just made from Professor Herbert, which he doesn't want anyone to know about. Herbert is a professor at a local college who teaches art history and classes on antiquities. He meets Nancy and seems like a normal guy though.
Bess, George, and Nancy stop by Todd's jewelry shop when he's gone and pretend to the clerk that they find jewelry making fascinating. He lets them check out the workshop while he helps a customer, and Nancy manages to find a mold to make a Golden Horse! She also learns that the letter Stone got came from the typewriter in his own office and discovers that he made the whole thing up.
While on the way home, they notice a tan car following them. A completely different car tries to run her and George off the road, but the cops rush to help, claiming that someone called in a report. That someone is Margaret Parker, who was in the tan car behind them. Margaret refuses to talk to Nancy and pretty much runs away.
Hoping to talk more with Herbert, Nancy stops by his office. He keeps talking about her necklace and ignoring all her other questions. When his secretary comes in with news about something he plans on selling, he rushes Nancy outside. On the spur of the moment, she steals the ribbon from his typewriter and rushes outside. She and George find that it's a note to someone named Sharp about setting up a deal to sell the real Golden Horse.
Before they can do anything smart like take it to the cops, they decide to sneak back into his office and look for the horse. They instead get caught by the professor who, for some strange reason, has two attack dogs with him. I repeat, he has two attack dogs standing guard over his office on a college campus in the middle of the night. He forces them outside and into his car, where he drives them to a secluded cabin.
College professor and two barely legal girls in a cabin in the middle of nowhere is not the start to amateur porn. It's really just an excuse for him to get rid of Nancy. After taking the necklace from her, he ties them up and sets the cabin on fire. He then makes the mistake of opening the necklace, which causes a red powder to come out and blind him. The two girls escape, but Nancy feels bad enough that she goes back inside and rescues him.
Once the cops arrest Herbert, he reveals the location of the real Golden Horse. It turns out that he authenticated the real thing, stole it, and then replaced it with plans to sell the original. The cops ask Nancy to go undercover and help them set up Sharp, the go between. Nancy takes an envelope and meets with Sharp, who is actually Justin Todd. Realizing it's a set up, he forces Nancy in his car and drives off.
That's when the full story comes out. Todd and Herbert worked together to con Stone and steal the real Golden Horse to sell it. He then made a replica and put it back in the museum. Nancy convinces him that she's just a student Herbert asked to do a favor for in exchange for extra credit. When he takes her back to drop her off, the cops surround his car and arrest him.
In the end, it turns out that Stone found out the Golden Horse was a fake and forced Margaret Parker to help him steal it. He planned on using the insurance money to cover up his mistake and buy more artifacts, so he winds up arrested too. The museum board hires Hillary to be the new curator, and she can't wait to see the Golden Horse on display. She also hires back Su-Lin and her grandfather.
*Hillary is a suspect because she's so unhappy that her aunt left the family house to the city. Given that she's her aunt's last of kin and multiple generations of her family were born there, I can totally see her side of things.
*The museum has its own security guard, but he's some random old man who slept through the whole theft and is actually still asleep when the cops get there.
*At one point, someone breaks the window on her car and leaves a threatening note in the backseat with a knife through it. They basically just blow it off as another part of the job. It's weird too because it never gets mentioned again, and Nancy never mentions getting her car repaired.
*The red powder inside the locket is a type of chili powder that Tibetan people used for protection, which explains why it temporarily blinded Herbert.