Carson surprises Nancy with a cruise around the Great Lakes for her and her friends, and she surprises Bess and George by jumping out at them in a new bikini, seriously. After delivering the news, she suggests that they head to this new boutique where she got her swimsuit to get some new clothes for their trip. They meet Amber, another woman heading off on the same cruise. She explains that her boyfriend, Craig, works on the ship and got her a discounted ticket. They shop together and agree to meet up later.
Almost as soon as they board the ship, they see Craig and Amber in the middle of a big fight. Amber keeps complaining about him to her new friends because even though he's working, she thinks that he should spend more time with her. When Craig learns who Nancy is, he gets her alone and reveals a big secret. He's actually an Interpol agent working undercover on the ship to catch a jewel thief. Nancy agrees to help after seeing his badge and fills her friends in on the case.
The only problem is that this causes them to sneak around together a lot. Amber picks up on it and accuses the two of them of having an affair. She stops spending time with Nancy but does stick to Bess and George. Bess meets an older woman named Laura, who always wears a gold necklace and nail polish that matches her outfit. She encourages Bess to design clothing and says she'll even buy some of her pieces. When she comes to look at Bess's drawings though, she hates everything and acts like a bitch. The next time they see her though, she's completely back to normal and acting like their best friend. Bess and Nancy both wonder if she has an emotional disorder like split personality disorder.
At one shore excursion, Laura tells them to visit one particular shop and to check out all the amazing finds in the basement. While down there, someone locks them in a room and they almost don't get back to the boat before it leaves. Nancy also gets a note from Craig about how Amber is sick and wants to see her but she should take a specific set of stairs to get there. Bess goes in her place, falls down the stairs because someone left slick stuff there, and ends up on crutches for the whole book.
After another excursion, they come back to the boat to find Amber in tears. She tells them that she and Craig went out to dinner, he went to the bathroom, and just never came back. Since he's been acting weird and pulling away from her, she assumes that he dumped her, which might explain why he didn't even come back to the ship. Nancy calls her dad for help because now she wonders if Craig even works for Interpol.
Then, the other shoe drops. Amber comes to her and confesses the true story. Craig really isn't an Interpol agent, he just told her that to throw her off his scent. He and a friend case the ship to find the most expensive furs, purses, jewelry, etc. and steal it. Craig planned one last job and asked Amber to come with him. They agreed to meet at a restaurant after he walked off with the riches, but he never showed up. Since she had nowhere to go, she came back to the ship.
That night, a storm hits right above the ship. Nancy is snooping around outside when someone grabs her and tries to push her overboard. They succeed, but a wave moves the boat in a way that she just lands HARD on the floor below. Unlike other books, where she would just get up and brush herself off, this Nancy is actually so hurt that she can hardly move for awhile.
Laura tells the girls about all these delicious candy shops at the next stop and how she wants to check them off. Since she's acting weird again, the girls follow her but lose her when Amber tries to catch up to them and falls down. By the time they take care of her skinned knees and get her back to her room, Laura is waiting there with a gun.
I bet you're wondering what the hell is going on, right? Apparently, Laura is Amber's mother, and the two worked together to steal a bunch of diamonds, the same diamonds Craig wants. He really is an Interpol agent too. Amber arranged to meet him and make him fall in love with her, both to confuse him and to make sure that she was on the boat. I guess actually buying a ticket was way too hard. She met him at the arranged time, drugged him, and left him in an abandoned house with some food and water.
Amber, seeing all the boxes of chocolates, makes a comment about if those are the gifts for Uncle CN. They then tie the girls up and put them in the bathroom, saying that the porters will eventually find them but not until they get off the ship. They jam the door lock to prevent a key from working, which means the porters will need to remove the whole door to get inside. That happens much sooner than anyone expects, and the door opens to reveal both Carson and Craig.
Craig had a tracker in his watch, and when he didn't call Interpol at his appointment time, they tracked him down. He looked up Nancy's name and learned that she was that girl detective and the daughter of Carson, who is apparently super famous, so he called him. Nancy remembers the Uncle CN comment and realizes that the women are at the CN Tower.
Everyone rushes over there and finds the two about to sell the diamonds, which Laura hid inside boxes of chocolates, to some international man of mystery. Craig makes a call, the cops come, and everyone gets arrested. We also learn that Craig knew Amber was in on the crime the whole time and only “pretended” to fall in love with her. Oh, and the reason Laura acted so weird is that she was actually her much nicer twin sister who allegedly knew nothing about the crime sometimes. Since they're already there, Carson decides to treat everyone to dinner.
*Ned is in New York and doing an internship for an insurance firm. Ugh, how can someone so awesome work in insurance? He's on his way to Oklahoma City for a work trip and takes time out just to take a layover in Chicago so he can drive to River Heights and surprise Nancy with a bottle of expensive perfume. Aw.
*God love Nancy. Bess picks up on random things wrong with Laura like how she quit wearing her necklace and stopped wearing matching nail polish. Nancy's first instinct is a mood disorder or maybe split personality disorder before even considering it's two different women.
*George keeps going to the pool and tells them that she needs to stay in shape to make the swim team next season. Wait, what? Are they in high school and ready to start senior year, or are they getting ready to go to college? Since they keep taking trips like this, I assume they're out of school, but if it's already warm enough for swimming in the Great Lakes region, George should know if she made a college team already.
*When she tells George about Laura's reaction to Bess's designs, George points out that this is an adult woman with lots of money who maybe just doesn't like the clothing an amateur designed.
*They go out to dinner, so both Nancy and Bess wear elegant dresses, but George sticks with brown slacks and a striped button up shirt.
*George meets a professional football player on the ship and starts spending all her free time with him. Bess thinks they like each other, but George explains that he's married with kids and just likes talking about football. Um no. Professional athletes don't just hang around with hot teenage girls because they like talking about sports. To make it even worse, he gives her tickets to the next big game and says he can't wait to see her. Friends, my ass!