Mary Anne and the Secret in the Attic is the fifth book in the Baby-sitters Club Mystery series.
The kids at Stoneybrook Elementary School are working on projects for Heritage Day which gets Mary Anne interested in finding out about her own family history. She thinks about asking her dad, but decides not to because she knows asking about the past has always made him sad and since Richard has been so happy with Sharon, she doesn't want to ruin that by bringing up painful memories.
After a sitting job, Mary Anne goes through some old boxes of stuff in the attic and finds an old photo album with pictures of her parents' wedding and her baby pictures, but then gets confused when she sees a picture of herself with an old couple & it looks like she spent some time with them. Before Mary Anne can search anymore, Richard & Sharon come home.
Later, Mary Anne visits her mother's grave, but when she gets to the cemetery, she realizes that she doesn't know where her mother is buried. As she wanders around the cemetery for a while, Mary Anne stumbles upon Mimi's grave and has a good cry because if Mimi was still alive, she could've asked her for more information.
When Mary Anne comes home from the cemetery, she finds a bunch of letters from her grandmother, Verna Baker to Richard which states that he sent Mary Anne to live with her mother's parents in Iowa after her mother died and insisted that she returned. Mary Anne is shocked by this discovery and believes that her father didn't want her.
Over the next few days, Mary Anne starts acting unusual which sparks the suspicion of her friends. They assume that she had a fight with Logan, but that's not the case. Mary Anne decides to call Logan to tell him what's been going on, but when the phone rings and she goes to pick it up, she hears her father on the extension talking to Verna. Verna called to let Richard know that her husband, Bill recently passed away and she is pretty upset that he never had a chance to see Mary Anne again & wants Mary Anne to come to Iowa. Mary Anne is worried that she might be sent away again.
Mary Anne tells Dawn about what's been going on and Dawn tells her that she should talk to her dad for answers, but Mary Anne is reluctant to do so. Mary Anne decides to call Kristy & Claudia since their families were friends of hers and might be able to know something about Mary Anne being sent away when she was a baby, but the girls don't know anything about it.
Later on, the BSC gets together at the Schafer-Spier house to work on their own Heritage Day contribution. There is going to be a fair to raise money for the Stoneybrook Historical Society and the club will be running a photo booth; they'll be making cardboard cutouts of people from Stoneybrook's past.
While they're working on the project, the doorbell rings and Mary Anne answers it. She sees a very official-looking woman on the porch, who says that she wants to ask some questions. Mary Anne panics, thinking that the woman is a social worker checking up on her father's parenting abilities. With the house noisy & chaotic with everyone covered in paint, Mary Anne explains to the woman that they're working on an education project and that her dad doesn't usually let things get out of hand. The woman is confused and says that she'll call Richard to make an appointment. After the woman leaves, the rest of the BSC thinks that Mary Anne should talk to her dad.
After the Heritage Day picnic, Mary Anne decides to talk to Richard about everything that she's discovered from her past & grandparents and how she listened in on his phone conversation with Verna. She also tells him that she's worried that she'll be sent back to live with Verna after the visit from the social worker.
Richard explains to Mary Anne that Verna wants her to come to Iowa for a visit & the social worker was just a census taker. He explains to her the story of why Mary Anne was sent to live with her grandparents in the first place: Richard was devastated after his wife died that he couldn't take proper care of Mary Anne, so he sent her to live with the Bakers for awhile. The Bakers wanted the living arrangement to be permanent, but Richard had a fight with them to get Mary Anne back. When Richard finally did, he & the Bakers decided that it would be too painful for them to stay in contact with Mary Anne, so they cut off all contact with each other.
A few days later, Richard gives Mary Anne a letter that her mother wrote to her before she died. He was supposed to give it to her on her 16th birthday, but decided that now is a better time to give Mary Anne the letter. Mary Anne tears up after reading the letter and then decides that she would like to go to Iowa to meet her grandma.
Mary Anne's mother died when Mary Anne was just a baby. And since it makes Mr. Spier too sad to talk about her, Mary Anne hardly knows a thing about her mother. Mary Anne respects her father's feelings. But lately, she wants to know more. She has to find out about he mother.
So Mary Anne does something she knows is wrong. She goes up into the creepy, ghostly attic in her and Dawn's house to look for evidence.
And there Mary Anne finds something she wishes she never had...