Carson gets an invitation from an old friend who is now the captain of a cruise ship to come on a cruise to Argentina. From the second they arrive, Nancy uncovers proof of something weird going on. Nina is an older rich widow who has eyes for Carson, but someone leaves the woman a box of chocolates filled with black widow spiders. Nancy barely manages to get the spiders out of her room.
At dinner, she meets the hotel manager Antonio who shares a gruesome look with Nina. She also meets Randy, a hunk of a guy in charge of guest relations, a young married couple, and a woman named Lynne who clearly has an eye on Randy. Nancy overhears Antonio threatening Nina about a map her husband left behind, and she then sees the woman looking over blueprints of the ship.
When she tells her dad about everything, he thinks she's making things up and needs to just relax. Someone delivers a bouquet of flowers to her room with a warning that they might be her funeral flowers, and the ship's florist has no record.
An unseen person pulls a switchblade on her and almost kills her, and then someone else rigs up the lights in her bathroom to electrocute her. Since nothing stops her, she also sneaks into Antonio's room and finds a note he wrote with Nina's name on it, but he comes in, she hides, and when she comes out, the note is gone. She and Carson then go on a day trip and have fun until someone nearly runs her over with a jet ski.
Luckily, Randy is there to save her. He later tells her that he knows the person did it on purpose, and she tells him everything she found so far. Someone then shoots Randy with a dart laced with a jungle poison. Seriously. She swings by Nina's room and finds it completely empty, and Antonio later tells them that Nina had important business back home and left the ship early. They finally get to Rio and find Ned waiting for her. He agrees to help her on the case.
Turns out that Nina's husband once worked for the cruise ship. They find out that he was involved in the theft of some major emeralds right before he died and that he left a map for his wife to help her find the gems. Nancy and Ned sneak back onto the ship, find a copy of the map, and find some jewelry belonging to Nina. They then find Nina herself hidden in a life boat.
After making sure that she's still alive, they basically steal the life boat and row it back to shore. They get Nina to Carson and talk about everything. Nina tells them about her husband's involvement in the crime and how Antonio was his accomplice. She also admits that she was the one who pulled a knife on her because she thought Nancy was Antonio. They also determine that Antonio must be working with at least one other person but have no idea who it might be.
They somehow figure it out that the emeralds are hidden inside a chandelier on the ship. Nancy sneaks in, Antonio catches her and waves a gun around, and Ned comes out of nowhere and whacks him from behind. They decide to split up, which leads Nancy to run into Randy. He claims the poison was such a low dose that he didn't even need to go to the hospital, but she sees some of the poison on his jacket and figures out that he was involved.
Though she makes up an excuse to get rid of him, she sees him following behind her. Nancy gets on a cable car, only to have Randy follow her on. He pulls a knife on her, they fight, and as the car moves up some random mountain (exactly how it's described in the book!), he opens the doors and pushes her out. Luckily, she manages to grab onto a safety bar at the last second.
It turns out that Ned saw them leaving and jumped on the roof, so he pulls her to safety. They then get back inside the car and knock out Randy. When he comes to, Ned literally puts him in a full nelson wrestling move. We then get to hear Nancy sum up everything, including how it was Randy's idea to steal the gems in the first place. Ned then takes her for a walk on the beach, asks if she thought Randy was good looking, and she just laughs and tells him to kiss her.
*Carson is completely unlike himself in this book. He tells her repeatedly that there is no case and that she needs to just leave things alone.
*Her dad also ditches her multiple times to do something with Nina, even though it was his idea to take a family vacation together. When she tries to do things on her own, he complains that she's never around. Um maybe your daughter doesn't want to see you hitting on women around her?
*I LOVE Ned! He's so cute in this book. He does this whole playful thing about how her cases mean more to her than he does and jokes around with her about how they need to take their own vacation together.
*You have to love that Nancy pretty much cheats on Ned in every book and no one ever calls her out on it. While she later grows suspicious of Randy, her first impression is that he's really hot.