The District Attorney, Bill Samuels, who has a cowlick that causes him to resemble Alfalfa from the Little Rascals series, tells Anderson to break Maitland's alibi in order to turn a long court battle into an open-and-shut case. Anderson discovers, however, that not only do multiple eyewitnesses confirm Maitland was out of town when the murder occurred, at a writer's conference in a neighboring fictional town known as Cap City, but security footage provides confirmation as well. With the assistance of private investigator Holly Gibney (a character from King's novel Mr. Mercedes), Anderson begins to dig deeper into the case, following a trail of clues that takes the duo to the fictional town of Marysville, Texas. Jack Hoskins, Ralph's rival in local law enforcement, is coerced by the Outsider (the real killer) to kill Ralph, promising that if he kills Ralph he will save him from cancer. With the help of Oklahoma State Police detective Yune Sablo (a Mexican American), Claude Bolton, an eyewitness to the Maitland Case and the next victim of the Outsider, and Claude's mother, Marysville resident Lovie Ann Bolton (a white woman in the predominantly Latino town), Anderson and Gibney discover that they have an actual monster on their hands, and set out to destroy him in a large cave, a tourist attraction known as The Marysville Hole, where two children died years earlier.^[3]

The novel is told through narratives alternating between two periods, and is largely told in the third-person omniscient mode. It deals with themes that eventually became King staples: the power of memory, childhood trauma and its recurrent echoes in adulthood, the ugliness lurking behind a façade of small-town quaintness, and overcoming evil through mutual trust and sacrifice.

King has stated that he first conceived the story in 1978, and began writing it in 1981. He also stated that he originally wanted the title character to be a troll like the one in the children's story Three Billy Goats Gruff, but who inhabited the local sewer system rather than just the area beneath one bridge. He also wanted the story to interweave the stories of children and the adults they later become.^[1][2]

The novel won the British Fantasy Award in 1987, and received nominations for the Locus and World Fantasy Awards that same year.^[3]Publishers Weekly listed It as the best-selling book in the United States in 1986.^{[citation needed]} It has been adapted into a 1990 two-part miniseries directed by Tommy Lee Wallace, and into a two-part film duology directed by Andy Muschietti; It: Chapter One was released in 2017 and It: Chapter Two will be released in 2019.^[4][5]