Bess and George call Nancy for help after Bess's new locket goes missing. She took it off to jump rope and asked George to hold it for her. George instead put it in her backpack and when she came back, it was gone. Someone left a sandwich with mustard, ketchup, relish, and peanut butter in its place. George gets distracted and wants to show off her picture from the paper, which leads Bess to saying that she'll never speak to her again.
The two girls get into a huge fight that ends with Bess tearing up the picture and the two storming off. Nancy sticks around to make up her suspect list, which includes this mean girl named Brenda and another girl named Karen. She suspects Karen even more after seeing her riffling through a coat that isn't hers. Karen claims she just wanted a piece of gum from her friend's jacket.
Bess agrees to help investigate the case as long as George isn't around. She gives Nancy a list of all the kids who were around that day, which includes Karen and some kid named Mike. Nancy decides to call all their suspects to find out what they eat for lunch. She learns from Karen's mom that she'll eat anything if it has peanut butter on it. Nancy then gets George to help her spy on Karen. George agrees as long as Bess isn't around.
They find absolutely nothing except that someone closes the curtains when they keep staring at the house. Karen's little brother comes outside and talks to them. He tells them that Karen hates relish and won't eat it, so Nancy takes her off the list. Brenda, who is a total brat, says she found Bess's necklace but then takes them to a random piece of rusty chain. She then laughs and says it looks like something Bess would wear.
One of the more unique suspects on her list is poor Ned Nickerson. George and Nancy follow him around until he spots them and asks what they want. She asks him random questions about lunch and what he eats. He waivers between being a smart ass and being nice, so Nancy isn't sure if she likes him or not.
The whole class goes to a pumpkin farm for a field trip. Nancy gets tired of George and Bess fighting all the time and tells them that she won't have lunch with either of them until they make up. They finally do make up, but when Nancy goes to grab her lunch, she finds hard boiled eggs. That's weird because she hates eggs and Hannah promised her something special. It turns out that she and this girl Jenny have the same lunchboxes and switched them. That makes her realize what happened to the locket.
It turns out that George had the wrong backpack. She accidentally switched with someone and never noticed that she had the wrong one because she never opened it over the weekend. They open it and find that she has Ned's bag. After making the switch, Nancy asks about the sandwich. Ned tells her that his uncle did it as a joke. Bess and George then really make up and apologize for being mean to each other.
*George is kind of horrible in the beginning! She scored the winning goal for some big soccer game and got her picture in the paper. While Bess is crying and super upset, she just wanders off and starts showing some girls her copy of the paper. Nice friend/cousin!
*Bess's Aunt Sarah gave her the locket. It's real gold with her name engraved on the front and an actual pearl. Who gives something like that to a kid in second grade?
*Nancy's favorite outfit is jeans with a pink sweater and matching pink sneakers. That is super adorable.
*Ned is in fourth grade in this book and is the first person George knows who rides a bike with gears.