The Sinclairs spend every summer on their private island, Beechwood. Born within a few short months of each other, 3 of the Sinclair grandchildren and one friend of the family spend the summers thick as thieves, calling themselves The Liars. Cadence, Johnny, Mirren and Gat are inseparable every year, but the summer of their fifteenth year, things start to change. Cadence begins to fall in love with Gat (a friend of the family who is invited to Beechwood every summer) while the Sinclair family falls apart around them.
The summer ends in a tragic accident when Cadence is found in the shallow water of the beach, unconscious and on the verge of hypothermia. Unable to remember what brought her to the beach Cadence spends most of the school year recovering from her injuries and suffering severe migraines brought on by head trauma (the doctors tell her she must have hit her head on the shallow rocks along the beach).
Cadence’s sixteenth summer she does not join the liars at Beechwood but is instead sent on a European tour with her father, spending much of the time bedridden from migraines. It isn’t until two years after the accident that Cadence returns to Beechwood and The Liars. Meanwhile her grandfather has built a new house, and family relations seem at an all-time high, Cadence’s accident ending her families constant bickering.
But things have changed between Cadence and The Liars since the fifteenth summer. Gat is despondent, Johnny is hiding something and Mirren fights bouts of nausea. Something isn’t right, and it has to do with her accident. Cadence is determined to find out the truth about The Liars, Beechwood and their fifteenth summer.