The following is a list of mysteries the BSC has solved, how they solved each one and what the solution turned out to be.
Mary Anne is looking in her attic for family pictures and finds several photos of herself as a very young child, with two strangers in a place she doesn't recognize. Later, she finds a bundle of letters that explain that the strangers are Mary Anne's mother Alma's parents, Verna and Bill Baker, and that Mary Anne's father Richard had sent her to live with them in Maynard, Iowa after her mother died, when Mary Anne was about six months old. When she continues reading, she finds that Richard had demanded that Mary Anne return to him about a year later. She then overhears a phone call from Verna, telling Richard that Bill had passed away and that although she and Bill had decided it would be better if they never saw Mary Anne again, Verna had changed her mind and wanted to see her granddaughter. Richard refuses to send Mary Anne to Maynard. The next weekend, the BSC is working on a project for Heritage Day at Mary Anne's house when a woman comes to the door whom Mary Anne assumes is a social worker, coming to see whether Richard was fit to take care of his daughter; Mary Anne is upset at how chaotic the house appears, especially Logan walking around shirtless because of paint on his clothes. She breaks down after the woman leaves, thinking that Verna is going to sue Richard for custody of Mary Anne, and the BSC convinces her to talk to her dad about the situation. She finally confronts Richard about it; Richard tells her that the woman was a census taker and that Verna is only asking for Mary Anne to visit, not to stay with her permanently. He also tells Mary Anne that he only sent her away because he was too sad after his wife's death to take good care of her. Mary Anne goes to visit her grandmother and has a good time, learning a lot about her mother while she's there.
Claudia notices that her sister Janine is acting weird: she's wearing make-up and borrowing Claudia's clothes, she seems distracted and is running late all the time, and then she lies to her parents about being at the library when she's really going to Pizza Express. Claudia, Mary Anne, Derek Masters (who's just done a guest appearance on the TV show Kid Detectives) and Derek's brother Todd follow Janine and see her meet a teenage boy at Stoneybrook High School. Later, Claudia follows her again and sees her get into a car with the boy; at home that night, Janine tells their parents she was working at the college chemistry lab. Worried about why Janine might be lying, Claudia tells their parents what she saw, and they confront Janine, who confesses that the boy Claudia saw is Janine's secret boyfriend, Jerry Michaels.
Someone is starting fires by burning books at the Stoneybrook Library during its Readathon fundraiser. After the BSC investigates several possibilities that turn out to be dead ends, Mary Anne realizes that all the books being burned are on the Readathon's fifth-grade reading list, and they're all being burned on Wednesday afternoons near their shelving locations. The BSC hides near a book on the list that's shelved near the only trash can in the children's room where a fire hasn't yet been set, and sees Sean Addison take the book and prepare to set it on fire. They confront him and Sean confesses that he started the fires because he was angry at his parents for ignoring him.