For as long as she can remember, Cadence Sinclair Eastman has spent the summer with her cousins and their families on Beechwood, her grandfather's private island near Martha's Vineyard, Massachusetts. The Sinclairs are a wealthy and respected white American family who seem to be perfect, but Cadence's narration hints at the emotional repression and secret-keeping necessary to keep up appearances.
Cadence loves spending the summers at Beechwood, which features a separate house for each of the Sinclair daughters. Days fly by with her same-age cousins, Johnny and Mirren, and their friend Gat Patil, whose uncle Ed is dating Aunt Carrie. Cadence identifies each summer by the age that she was at the time. She recalls when Gat and Ed first came to the island for summer eight. Her grandfather and grandmother are visibly shaken by the surprise that Ed is of Indian descent. Gat's ethnicity isn't an issue for Johnny, Mirren, and Cadence, and the four quickly form a clique. To the adults, they become known as the Liars.
As a teenager, Cadence falls in love with Gat. She arrives late to the island for summer fifteen because she and her mother have been busy redecorating their home in Vermont to erase the presence of Cadence's father, who recently walked out on Penny for another woman. Cadence represses her grief over her father's abandonment, but her blood begins to pump again when she sees Gat. However, her hopes are dashed when she learns he has a girlfriend back in New York. Despite him already being in a relationship, Gat is affectionate with Cadence, who he eventually kisses. The two behave as a couple for the duration of the summer, never talking about his other girlfriend.
Summer fifteen is cut short when Cadence suffers an accident whose details she cannot recall. The rest of the year is hard for Cadence; she suffers from severe migraines and becomes addicted to prescription painkillers. She has to repeat the year at school, and when she reaches out to her cousins, she is ignored. Because Penny won't let Cadence go back to Beechwood for summer sixteen, Cadence accompanies her father on a tour of Europe. Cadence assumes that since Gat isn't contacting her, he must never have loved her. Cadence also starts a project: to give away one of her possessions every day.
Penny allows Cadence to go back to Beechwood for summer seventeen. She finds it has changed. Her grandfather's house on the island, Clairmont, has been completely replaced by a stark, modern architectural creation known as New Clairmont. Her grandfather is suffering from dementia. Cadence is happy to be reunited with the Liars, but they don't want to talk about summer fifteen, and they also don't want to spend time at New Clairmont, preferring instead to stay at Cuddledown, the home built for Aunt Bess.
Being at Beechwood begins to trigger some of Cadence's lost memories of summer fifteen. She remembers that her grandmother, Tipper, had died, and that her mother and aunts began viciously arguing over the disposition of their inheritances. Cadence notes how they are all powerless to make any decisions for themselves as grown women without the blessing of their controlling father. This is why Aunt Carrie refuses to marry Ed: she knows her father will not accept an Indian man becoming part of the family. Cadence also remembers that the adults tried to involve the Liars in their feud, making the situation increasingly acrimonious.
To teach the elder members of the family a lesson and to bring about a cleansing of the island, Cadence and the Liars plot to burn down Clairmont, a symbol of Granddad's patriarchal power. They believe doing so will set a different tone and bring the family back together. The Liars put their plan into action one night when all the adults and staff are off the island. However, Cadence recalls that, being drunk, she set fire to the main floor of the house too early. With the other Liars still dousing their floors with gasoline, Cadence was the only one to survive. She jumped in the ocean and hit her head, then wound up with burned hands in the hospital.
As the memory comes back, Cadence seeks solace in the arms of Gat. Gat gently tells her it was not her fault that they died, but he also lets her know they will not appear to her anymore. The novel ends with Cadence going to the shoreline with the Liars, who swim beneath the surface and disappear, leaving her alone on the shore.
