The Mystery at Claudia's House is the sixth book in the Baby-sitters Club Mystery series.
When Claudia comes from a sitting job one day, she notices her bedroom closet is open, her makeup is re-arranged and the papers on the desk have been moved. At first, Claudia thinks that she's been robbed, but the rest of the house appears normal and nothing is missing, but Claudia wonders who could've been in her room.
One morning, Janine comes downstairs for breakfast late because she was putting on makeup and comes home late for dinner a few times. She also wants to borrow Claudia's favorite red sweater. Claudia catches Janine coming out of her bedroom & she finds things rearranged again. Claudia wonders why Janine is so interested in clothes & makeup all of a sudden.
When Claudia tells the rest of the BSC about Janine's unusual behavior, Mallory thinks that Janine might have a boyfriend. One day, Claudia follows Janine over to Stoneybrook High School (while babysitting Derek Masters who is back in Stoneybrook for awhile) and watch her meet a very cute guy.
Janine continues to lie about her whereabouts, so Claudia, Derek, and Todd come up with another plan to get Janine to confess; they pretend that Derek has to set up a fake courtroom for school and they need Janine's help with the legal stuff. They talk about how important the truth is when Derek confesses that he is supposed to kiss a girl on his TV show, but he has never kissed a girl before & told all of his friends that he had.
One afternoon, Claudia follows Janine again and sees her get in the car with the cute guy from the high school. Claudia gets concerned especially when she hears Janine lie to her parents about where she was that afternoon.
Thinking that Janine might be in trouble, she tells her parents who call a family meeting. Janine confesses that the guy she was with was her boyfriend. She was lying because she wanted to keep him to herself for a while. Janine is mad at Claudia for telling, but they later make up after Janine's boyfriend, Jerry Michaels comes over to the Kishi house for dinner.
Something odd is going on at the Kishis'. First Claudia's room is ransacked. Then she discovers some of her things missing. What would someone want with Claudia's old stuff?
Weird things have been happening with Claudia's sister Janine, too. Janine has been getting suspicious packages in the mail, and she even got caught lying to their parents!
Claudia doesn't know what's going on at her house. But things are going to change with the Baby-sitters on the case!