Every year, three generations of Cadence's family spend their summer on Beechwood, their private island off the coast of Martha's Vineyard. It's her favorite place, largely because of The Liars: her cousins, Mirren and Johnny, and her best friend, Gat. The year they are all sixteen years old, they arrive on the island expecting the usual three months of mischief, sandcastle contests, and lemonade – but everything changes when Cadence and Gat begin to develop new, intense feelings for each other, and tensions between the adults on the island threaten to expose family secrets. Caught in the bliss of first love, Cadence doesn't yet know that her fairy tale summer will end with her alone, washed up on the beach with a traumatic brain injury -- and no memory of how she got there.
 

Having newly remembered a family tragedy, Cadence convinces the Liars to tell her about the aftermath. Mirren confides in her about an encounter with a crush gone awry, while Johnny remembers a rival's unwelcome visit to Beechwood. Gat has no choice but to recount a painful turn in their relationship – one that threatened their long-standing friendship and their potential new romance. And they all bore witness to brewing tension between the aunts, Carrie, Penny, and Bess. But as they tell their stories, at least one of The Liars is hiding something from Cadence, fueling her suspicions that last summer's pivotal trauma may not have been an accident after all.