We Were Liars is a 2014 young-adult novel by E. Lockhart, a pen name of writer Emily Jenkins. The book tells the story of Cady (short for Cadence), who is nearly 18, as she recounts the story of her life. She is a member of the wealthy New England Sinclair family whose patriarch and matriarch, Harris and Tipper, still dominate the family as the story begins. The Sinclair family is so wealthy that it owns an island called Beechwood with four houses—Clairmont, Cuddledown, Windemere, and Red Gate—that are shared by three sisters—Penny (Cady's mother), Carrie, and Bess. Much of the story takes place during summers spent on Beechwood Island with Cady's friends: Johnny, Mirren, and Gat. These four are "the Liars" of the book's title.

Cady has known Johnny, Mirren, and Gat since they were youngsters. Gat is South Asian and the son of Ed, who is the new boyfriend of Cady's aunt Carrie. With time, Gat and Cady fall in love. In the background of their lives, the aunts fight over who will inherit their parents’ property, which includes a very large and valuable house in Boston. Their fighting is made worse by the sudden death of Tipper. 

During the summer of her fifteenth year, something bad happens to Cady. She is found on the beach unconscious, and she cannot remember how she got there. During the two years that follow, she is sick and needs lots of medical attention. Her head hurts constantly, and she has to take strong pain relievers to make the migraines go away. She takes a trip to Europe with her father, but her illness continues. She can’t remember what happened to her during that fateful summer. She writes to her friends, the Liars, but none of them respond. Gat, despite his professed love for her, simply disappears from her life. Cady is puzzled and hurt.

When she’s 17, Cady finally feels well enough to return to Beechwood for the summer. Her mother is very controlling and tells her she has to eat dinner at Clairmont each evening. She also refuses to tell Cady what happened to her during the summer of her fifteenth year. The same is true of everyone else in the family. When she asks what happened, the family claims that the doctors advised them that it would be better if she recalled herself what happened. With this large mystery in the air, Cady rejoins Johnny, Mirren, and Gat, who are camping out at Cuddledown and keeping apart from the adults. Cady and Gat reignite their love, and it is as if nothing ever happened to disturb it. 

With time, Cady begins to remember the events of “summer fifteen,” as she calls it. First, she remembers that she became so angry with the aunts for arguing over the division of the family property that she inspired the Liars to burn down Clairmont, where Harris, the family patriarch, was living at the time. She became disgusted with how Harris was manipulating his daughters, making them compete for his affection and his property. She became equally dismayed by the behavior of the aunts, who had come to seem petty and greedy in her eyes.

But, as it turns out, that is not the whole story. Cady also recalls that her good friends, all three of the other Liars, died in the blaze and that she was likely responsible. While they went to the basement and the upper stories of the house to pour gas and throw matches, she lit the fire on the first floor. However, she did it too soon, before her friends could escape the conflagration. They died in the blaze, and have re-appeared to her as ghosts.

This is the great tragedy that no one wants to tell Cady, and she finally comes to recall it herself. She is greatly saddened, but she derives good lessons from the events about how to lead one's life.