With a corporate job, a steady relationship, and a sick mother, Fred's (Dylan O'Brien) chance encounter with a man from his youth leads to frightening memories. Little by little, he unravels a mystery that has been hidden for years about a missing girl, a drug called Mercury, and a terrifying creature who has accompanied him every step of the way. Past, present and future intertwine, and Fred begins to explore all the possible lives he could lead. Which one will he choose?