We Were Liars is a young-adult fiction novel by E. Lockhart written in 2014. The novel promotes a message of self-acceptance after tragedy and the importance of familial support. It centers around the Sinclair family and is set on their private island, Beechwood, located near Martha's Vineyard. The protagonist of the story is Cadence Sinclair Eastman, the oldest grandchild of the Sinclair family's wealthy patriarch, Harris Sinclair.

The novel begins in the year Cadence is fifteen; hence, she refers to the summer as "Summer Fifteen" throughout the novel. During this summer, Cadence spends time with her cousins, Mirren and Johnny, as well as the nephew of Cadence's aunt Carrie's partner, Gat, with whom she develops a romantic relationship. The four of them—Cadence, Mirren, Johnny, and Gat—are referred to as "The Liars" by the other members of the family (hence the title of the book). Toward the end of the summer, though, Cadence suffers a serious injury to her head during an accident in the water. She loses most of her memories from Summer Fifteen in the aftermath of this injury and also begins to suffer from chronic migraines. As she tries to heal from this injury, she also develops an addiction to Percocet.

The following year, in an effort to help her daughter recover from the trauma of the past year, Cadence's mother encourages her to spend the summer with her father in Europe rather than returning to Beechwood. Thus, Cadence does not return to the island until Summer Seventeen, and when she returns, she is surprised and quite disappointed to find that so much has changed. Her grandfather, Harris, now suffers from severe dementia, and his beautiful and historic estate has been remodeled into a cold but modern structure.

Cadence tries to reconnect with "The Liars" but finds them to be acting strangely, refusing to talk about what happened to her in Summer Fifteen. However, the group does connect over a shared hatred of the new look of their grandfather's estate, and so they decide not to stay there and instead camp together at a nearby location.

As the time progresses, memories from Summer Fifteen begin to return to Cadence. She remembers her mother and aunts continually fighting and arguing over their inheritances, and she recalls how uncomfortable she and The Liars felt about this bickering. She also remembers proposing an idea to The Liars in Summer Fifteen in an attempt to stop all of the fighting: the thought was to burn down their grandfather's estate, hoping it would stop the family from fighting over the property. At first, she is happy about this memory, thinking she was able to keep her family together, but then the rest of her memories begin flooding back.

Cadence remembers that she and The Liars forgot to let out their grandfather's dogs before lighting the estate ablaze, and she recalls that the dogs were killed in the fire. Gat then confesses to her that the dogs weren't the only casualties; Johnny, Mirren, and Gat all died in the fire as well. At this point, Cadence realizes that she has been spending time with either the spirits of her cousins or hallucinations of them, and the place she is camping with them is actually the site of their burial. The Liars assure her that their deaths were not her fault, but they also tell her that they will no longer appear to her after this point. Cadence spends a few days grieving the loss of her cousins and friends before finally returning to her family. The story ends with Cadence determined to finally move on and begin her life anew.