Stacey and the Missing Ring is the first book in the Baby-sitters Club Mystery series.
Stacey wants to have a diamond birthstone ring, but her mother thinks it's too expensive and won't buy it for her. When Stacey goes to the mall with the rest of the BSC, she sees an even more expensive birthstone ring at a jewelry store.
At the next BSC club meeting, they get a call from some new clients, The Gardellas, who need a babysitter for their 7-month-old daughter, Tara. Stacey gets the job to babysit for Tara.
When Stacey arrives at the Gardellas, she thinks Mr. & Mrs. Gardella are a bit weird for fussing over their cat, Mouse, and dog, Bird more than their baby, but overall, the job goes well. Early the next day, Mrs. Gardella calls Stacey to tell her that her diamond ring is missing & accuses Stacey of stealing it. She also says that she'll be calling other clients of the BSC to let them know what happened. After Stacey calls Kristy about what happened, and Kristy schedules an emergency club meeting for that afternoon.
At the meeting, Stacey claims that she didn't steal the ring and the rest of the club believes her, but some of the BSC clients don't and they end up getting less jobs and even cancellations. The Prezziosos end up leaving a dinner party early after meeting the Gardellas there because they believe in the BSC and were upset over what the Gardellas were saying about the club.
Claudia and Stacey get into a fight after Claudia searches Stacey's jewelry box for the ring. At the next club meeting, Stacey decides to sit for the Gardellas for free until the ring is paid off, but the other members don't think that Stacey should have to repay Mrs. Gardella for a ring she didn't steal, but Stacey doesn't know what else to do. She calls Mrs. Gardella, who agrees to it only if Stacey brings another sitter with her.
When Stacey sits for the Gardellas on Friday, she takes Kristy with her. During the sitting job, they notice Mouse, the family cat taking all of his toys out of his room one by one, & then returning without them. When Stacey follows him, she discovers a couple of hiding places where there's toys, paper clips, and Mrs. Gardella's ring.
When the Gardellas get home, the girls show them where the ring was. Mrs. Gardella apologizes to Stacey and says that she'll call the Prezziosos to clear things up as soon as she can. When Stacey asks her to call the other clients as well, Mrs. Gardella says she only told the Prezziosos and the BSC's slow period was unrelated. In the end, everything is back to normal.
Stacey is shocked when a new family accuses her of stealing a valuable ring from them when she was baby-sitting. Stacey would never take anything from anyone!
Even worse for the Baby-sitters, the Gardellas are threatening to tell their other clients about the missing ring.
Will everyone in Stoneybrook think the Baby-sitters are stealers? Not if Stacey can help it. She's going to find out what happened to that ring!