Mike Lawford, a New York literature professor, is devoted to his young son Charlie. On Halloween night, Mike takes Charlie to a carnival where Charlie mysteriously disappears after asking, "Daddy, can we pay the ghost?" The police are unable to find him, and the incident strains Mike's relationship with his wife Kristen. Over the next year, Mike becomes obsessed with finding his son, delving into books and old records while experiencing disturbing visions. Despite being dismissed by authorities and friends, he follows a trail of cryptic clues that point to an ancient and malevolent force.

As Halloween approaches once more, Mike's relentless search uncovers a dark history behind the annual festival and a pattern of disappearances that span centuries. He learns that he must confront and understand the supernatural entity responsible if he ever hopes to be reunited with Charlie. Digging deeper into folklore and enlisting the help of those who have encountered similar phenomena, Mike discovers he is in a race against time to avert an even greater tragedy.