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.

Vowing to get her sister Gemma back from the siren sisters no matter what the cost, Harper must face dangers unlike any she's ever experienced. Fortunately, she has Daniel by her side, a gorgeous guy who's devoted to helping her find her sister--and who's immune to the girls' dark powers.