Cadence Sinclair Eastman has spent every summer of her life on Beechwood Island, which her grandparents own and have built houses for each their daughters and their families. Clearly this isn't a family that is kind of wealthy, they (at least the grandparents) have loads of money. Cadence loves her summers on the island, mostly spent with the two cousins closest in age to her, Johnny and Mirren, and Gat (Johnny's almost step cousin) - this group of four is also referred to as The Liars. But on during summer fifteen something happened, which resulted in Cadence going for a swim, alone, at night, which she has no real memory of, but was found shivering on the sand with "most likely some kind of head injury, though the brain scans turned up nothing." After missing her sixteenth summer at the island Cadence returns for the seventeenth, grateful to be reunited with the liars, and also attempting to slowly remember the events of the fifteenth summer.
The Sinclairs are a family where everyone is perfect and put together and life is good (even when that really isn't the case at all). It's clear that something definitely not good happened during Cadence's fifteenth summer, something that has shattered the family a bit, however since Cadence can't retain any memories from that summer (even though she's apparently been told more than once) the reader is left in the dark as well. We Were Liars gets in your head (in a good way) and leaves the reader guessing what really happened to the end.