Charlotte’s mother died when she was young. She and her father have moved every two to three years for his job. When Charlotte and her father move to Brookline, Massachusetts, she is apprehensive about attending a new school.
They rent part of a house that has a beautiful tower, though the tower is off limits to them. The landlord lives in the basement and is rarely seen by anyone.
All seems to go well for Charlotte on her first day of school until lunch. Mrs. Rodriguez, her homeroom teacher, believes in class unity and assigns seats at the various lunch tables. From the start, Charlotte does not like the girls at her table.
One is named Katani. She is the sister of the star of the girls’ basketball team. This fashion diva immediately disapproves of how Charlotte looks. Another named Maeve is a strikingly confident, redheaded girl. She is spoiled and rich. She shows off her fancy laptop. And then there is Avery, a star soccer player. She acts like a human jumping bean.
After Charlotte makes a quick trip to the restroom, she reaches her place at the table before she realizes she forgot to zip up her pants. When she does zip them, she unknowingly catches the tablecloth in her zipper. When she stands to throw away her garbage, she pulls all the lunches to the floor. Her tablemates are upset with her, and all four girls become instant enemies.
Katani requests a table assignment change. The others want to change tables, too. Mrs. Rodriguez agrees to it under one condition: The girls must survive a weekend sleepover together. If they do, they can all move to new tables.
For a sleepover, they all need to get permission from their parents. Charlotte’s father happily grants it. Then he offers their home for the sleepover.
Katani’s grandma is Mrs. Fields, the principal of their school. As she drops off two of the girls, she asks about the upstairs tower. Unable to get answers about it, Mrs. Fields leaves.
It doesn’t take long for the girls to unpack and agree how to pass the time so they can have their new lunch seats at school. But before long, Katani taunts Maeve about being stuck up and spoiled. Maeve accuses Katani of being mean. Katani accuses Maeve of always showing off her new laptop.
The laptop turns out to be school issued to help Maeve offset her dyslexia. The girls find out that Maeve's life is not as charmed as they think. Her parents are overbearing, she puts in many hours outside of school with tutors, and she has an obnoxious brother who makes her life miserable.
Katani confides that her constant comparisons to her sisters and their athletic abilities wear on her. She is adopted, but everyone expects her to be just like them. In addition, she protects her younger sister, who is autistic.
As the group turns on Avery for struggling to absorb all of their “imperfections,” Avery is brought to tears. She struggles socially. Charlotte realizes that her perception of each girl was wrong. Slowly they change from enemies to friends.
The girls find a button on the wall. Maeve clicks it before Charlotte can caution her not to. The button activates a ladder that emerges from the ceiling. The girls climb the ladder into the mysterious tower. Charlotte knows she shouldn't be there, but she likes that these new friends think this house and tower belong to her.
The group adopts the tower as their new secret meeting place. Over the next few months, they spend almost every day fixing up and relaxing in this shared new space. One day they are goofing around in the tower, and Avery accidently knocks a floorboard loose. As she tries to fix it, she discovers an old piece of parchment and a jeweled key beneath the floor.
The parchment holds an oath of loyalty and a list of rules for the BSG. They don’t know what BSG stands for, but they are inspired to add their own twist to the existing oath and rules. Katani agrees to be the keeper of the special key and wears it around her neck for safekeeping.
When Katani's grandma sees the key that night, she almost drops a pot of potatoes. She asks her granddaughter a ton of questions. Katani doesn't like being interrogated, so she hides the key in her room and decides to put it back in the tower at their next sleepover.
Everything goes well until Charlotte’s father discovers that she and her friends are using the tower. He is upset that she has betrayed the rules of their rental agreement. The full truth is that Charlotte hasn’t simply betrayed her father; she has led her friends to believe the house belongs to her family.
They had no idea they were trespassing. They decide not to be Charlotte's friend because she lied to them. Charlotte is devastated that she has broken trust with all the people who are closest to her.
Early one Saturday morning, Mrs. Fields visits their home, but instead of wanting to see Charlotte's father, she yells, “Sapphire.” Charlotte is speechless. Mrs. Fields knocks on the pantry door. To her surprise, a small woman emerges from behind a hidden door.
The two women know each other. Mrs. Fields explains that Sapphire Pierce, the owner of the home, is her first best friend from when the women were young. It was during an era in our nation's history when most students at Abigail Adams School were white. Mrs. Fields, being African American, forged a deep friendship with Sapphire Pierce, who is half-Chinese.
Miss Pierce reveals that she knows the girls were in the tower. Charlotte is mortified, but Miss Pierce found their antics heartwarming. Miss Pierce devises a plan with Charlotte and Mrs. Fields to invite the girls over in an effort to work things out.
The girls are resistant but go to Charlotte's house. When they arrive, there is a note on the door that invites them up to the tower. They are surprised and proceed hesitantly. The tower is decorated with beautiful lights. Charlotte has hung them. Each of their names is displayed in the lights.
Charlotte explains everything. Miss Pierce comes up to meet all of the girls, and they get their second surprise when they learn that Mrs. Fields has known Miss Pierce for years. The girls all have the same question: What does BSG stand for. Beacon Street was the name of the street that connected Mrs. Field’s home to Miss Pierce’s home many years ago. Miss Pierce’s father nicknamed them the Beacon Street Girls because of all the time they spent going back and forth.
The best news of all is that the tower is theirs to share and enjoy together. From this day forward, these four girls have a special bond. The new BSGs couldn’t be happier.