Big Little Lies by Liane Moriarty begins with Mrs. Patty Ponder observing a riot happening across the street at Pirriwee Public. The school has just held the annual costume party/trivia night, and several of the fathers of the students are shouting and shoving one another. The chapter cuts away to a group of interviewees giving statements to a police detective. The parents are discussing how the school was divided over something that was being called “The Mummy Wars.” The last statement is from a police detective, talking about how they are investigating a murder.

The book then cuts away to several months before the trivia night where the readers are introduced to the cast of characters. The three main protagonists are Madeline, Jane, and Celeste, all mothers that have children attending kindergarten at Pirriwee Public. Madeline is a very outspoken mother who’s just turned 40. She has two younger children and a teenage daughter, Abigail. Abigail and Madeline have a strained relationship because she wants to move in with her father and Madeline’s divorced husband, Nathan, and his new wife, Bonnie.

Jane is the youngest and newest to town. She’s running away from a mysterious past with her son, Ziggy. Jane is very secretive over who Ziggy’s father is, simply saying that he was someone that she had a one night stand with several years ago. The group is rounded out by the beautiful and rich Celeste, who is the mother to twins, Max and Josh. Everyone in town looks up to Celeste because of her looks and success. Celeste is married to Perry, who is the breadwinner of the family.

The three mothers become friends at orientation Things get awkward when one of the mothers, Renata, accuses Ziggy of bullying her child, Amabella. Ziggy denies it, and Jane and her new friends stick up for him. Things get worse for Ziggy and Jane as Renata starts pushing a petition around the school trying to get Ziggy suspended for bullying. The school ignores it because there’s no actual proof of Ziggy bullying anyone, but the rest of the parents are forced to pick sides.

While this is going on, Celeste is dealing with a dark secret at home, that her husband Perry hits her. Celeste is too ashamed to tell anyone about the abuse and feels like she deserves it. As the group grows closer, Jane reveals that she knows the name of Ziggy's father. He was a man named Saxon Banks, who was abusive and humiliated Jane during their one night stand. Celeste is surprised to learn that Saxon Banks is related to her husband, Perry.

To make matters worse, Celeste discovers that it was her son Max who was responsible for bullying Renata’s child. That isn’t the only child that Max has bullied either. Celeste realizes that Max has seen her being abused and has started to pick up the aggressive behavior. Madeline tries to help her friends, but she has her own problems trying to keep her family together. Her daughter Abigail has decided to leave and live with Nathan and Bonnie. While she’s there, she starts getting interested in charity. In an attempt to raise awareness about child trafficking, she’s started a website where she’s trying to auction off her virginity.

Everything comes to a head as trivia night draws closer. Jane and Perry meet for the first time, and she realizes that he and Saxon Banks are one and the same. When confronted with the truth, Perry snaps and hits Celeste. In response, Bonnie pushes him over the edge of the balcony. She reveals that her mother was abused by her father when she was a child and she snapped when she saw the way Perry was acting. Everyone wants to protect her and keep the murder a secret, but Bonnie comes forward and confesses because she doesn’t want to see the families torn apart any further.

At the conclusion of the novel, Celeste has decided that her sons will not grow up to be the same kind of man as their father. She is in therapy to overcome her feelings of responsibility for Perry's death. Celeste uses her experience to help others by giving a speech on domestic violence. Her primary.message is that domestic abuse can happen to anyone.