In the quiet seaside town of Capri, sixteen-year-old Gemma enjoys a close bond with her sister Harper and a growing attraction to next-door neighbor with Alex, but everything changes when, after partying with newcomers Penn, Lexi, and Thea, Gemma discovers she has powers that will force a terrible choice.

With Penn and Lexi determined to kill Gemma and replace her with another siren, Gemma, aided by Harper and Daniel, must delve into her enemies' mythical past and darkest secrets in a last-ditch effort to protect all that she holds dear.