Though Holly is a standalone novel, it draws on the events of King’s previous books. Protagonist Holly Gibney first appears in King’s 2014 novel, Mr. Mercedes, part of the Bill Hodges trilogy. In Mr. Mercedes, private investigator Holly teams up with retired police detective Bill Hodges to hunt a rampage killer named Brady Hartsfield who murdered Holly’s cousin Janey. Holly joins the investigation along with Jerome Robinson, a 17-year-old who does odd jobs at Bill’s agency. They are instrumental in apprehending Hartsfield, and both receive Medals of Honor from the city.

In Finders Keepers, Holly joins Bill in investigating Morris Bellamy, a petty criminal turned murderer. In End of Watch, Holly begins working with Bill at his private investigative agency, Finders Keepers. After Bill is diagnosed with pancreatic cancer, Holly inherits Finders Keepers after his death. In the standalone novel The Outsider, Holly is hired to investigate the murders of several children in Oklahoma. She discovers that the murders were committed by a mysterious, inhuman presence which she dubs “The Outsider.” The Outsider can mimic the appearance of people, planting evidence to frame others for its crimes. At the end of the novel, Holly defeats The Outsider.