On a Caribbean cruise bound for its final destination, 42-year-old Nanna invites her daughter Cille and best friend Lea to celebrate Cille's 18th birthday, setting the stage for a dream vacation that quickly descends into nightmare. As Nanna confronts her suspicions about her daughter's traumatic experience, she takes matters into her own hands, leading to a thrilling and twist-filled ride that culminates in a shocking confrontation with the perpetrator. With a tight little melodrama that hurtles forward at breakneck speed, Birthday Girl is a psychological thriller that explores the complex relationships between mother and daughter, and the devastating consequences of sexual assault.