A misfit group of unwitting high school students stumble upon a cursed object, an ancient Aztec Death Whistle. They discover that blowing the whistle and the terrifying sound it emits will summon their future deaths to hunt them down.

Fifteen years after stabbing a classmate to conjure an imaginary phantom known as Mercy Black, Marina Hess is coming home. She's being released from psychiatric care to live with her sister and young nephew. But in the years since her crime, the myth of Mercy Black has gone viral inspiring internet rumors, stories, and even copycat crimes. Marina is haunted by what she has done and the phantom she imagined. Though she would rather leave the past buried, her nephew becomes increasingly obsessed with Mercy Black. To save him, Marina must face her past and uncover the truth behind Mercy Black. What she discovers is a very real and very deadly horror that will stop at nothing to claim her and her nephew.