1957–1958During a heavy rainstorm in Derry, Maine, a six-year-old boy named Georgie Denbrough sails a paper boat his brother made for him along the rainy streets before it washes down into a storm drain. Peering into the drain, startled from seeing a pair of glowing yellow eyes, Georgie encounters an eccentric clown who introduces himself as "Mr. Bob Gray," a.k.a. "Pennywise the Dancing Clown." Georgie declines a balloon but is enticed by Pennywise to reach into the drain and retrieve his boat, the clown ripping the child's arm off before leaving Georgie to die from bleeding out. His body is later discovered and brought back to the Denbrough's house.
The following June, an overweight eleven-year-old boy named Ben Hanscom is harassed by a bully named Henry Bowers and his gang, escaping into the marshy wasteland known as the Barrens when attacked by his tormentors on the last day of school. There, Ben befriends an asthmatic hypochondriac named Eddie Kaspbrak and "Stuttering Bill" Denbrough, Georgie's elder brother who suffers from a stutter and rides on a rusty bike named "Silver." The three boys later befriend fellow misfits Richie Tozier, Stanley "Stan" Uris, and Beverly Marsh, who eventually refer to themselves as "The Losers Club." As the summer draws on, the Losers each encounter Pennywise in terrifying manifestations: a mummy on a frozen canal to Ben, a fountain of blood (that only children can see) from Beverly's sink, a diseased and rotting leper to Eddie, drowned corpses to Stan, and a frightening phantom of Georgie to Bill. The Losers eventually realize they are all being stalked by the same child-killing entity which killed Eddie Corcoran in the form of the Gill-man.
Meanwhile, an increasingly unhinged and sadistic Bowers begins focusing his attention on his African-American neighbor Mike Hanlon and his father. Bowers kills Mike's dog and chases the terrified boy into the Barrens, where he joins the Losers in driving Bowers' gang off in a rock fight, a humiliated Bowers vowing revenge. Mike becomes a member of the Losers Club after revealing his own encounter with Pennywise in the form of a flesh-eating bird. While showing the Losers his historical scrapbook, the group realizes that "It" is a monster with a hold on the town. Following further encounters with It, the Losers construct a makeshift smoke hole that Richie and Mike use to hallucinate It's origins as an ancient alien entity that came to Earth in a meteor and feeds on children for a year before entering a 27-year-long hibernation.
In late July, after Eddie is hospitalized by Bowers and several of his friends, Beverly stumbles across the Bowers gang in the landfill, literally with their pants down, lighting farts with fire for fun. She hides behind a junked car and witnesses one of the bullies, Patrick Hockstetter, killed by It in the form of a mass of leeches while emptying a refrigerator he used to trap and kill injured animals. The Losers later discover a message from It written in Patrick's blood, warning them that It will kill them if they interfere. After Eddie is released from the hospital with a broken arm, Ben makes two silver slugs out of a silver dollar on the belief that silver will harm It. The story's narrative changes to Pennywise's perspective at that point, revealing It as a being from another reality known as the Macroverse that nourishes itself on fear from terrorizing children in the forms It assumes.
The Losers travel to the house on Neibolt Street, where Eddie, Bill, and Richie had previously encountered It, forcing Pennywise to flee into the sewers after Losers wound It with the silver slug while in the form of a werewolf. Deeming the Losers a threat, It provides Bowers with a switchblade while manipulating him into murdering his abusive father and recruiting his friends Victor "Vic" Criss and Reginald "Belch" Huggins into helping him follow the Losers into the sewers to kill them. But Vic and Belch are both killed by It in the form of Frankenstein's monster while a traumatized Bowers gets himself lost in the sewers. In the sewers, Bill performs the "Ritual of Chüd" he learned to face It in the Macroverse where he meets the monster's antithesis Maturin, an ancient turtle that created the universe (which it vomited up following a stomach-ache), who explains that It can only be defeated during a battle of wills.
Bill enters It's mind through the ritual and sees the true form of It, a mass of destructive orange lights called the "Deadlights" before Bill defeats the monster with Maturin's help. After the battle, not knowing if they killed It or not, the Losers get lost in the sewers until they take part in an orgy to bring unity back to the group. The Losers then swear a blood oath to return to Derry should It resurface. Bowers, having lost his sanity by the time he washed out of the sewers into a nearby river, is institutionalized after being blamed for the child murders.
In July 1984, at the annual Derry carnival, three youths brutally attack a young man named Adrian Mellon and throw him off a bridge. They are arrested and charged with murder when Mellon's mutilated corpse is found. One of the murderers claims that he saw an eccentric clown kill Mellon underneath the bridge. Adrian's boyfriend, Don Hagarty, the other victim in the attack, had also noticed the clown, but the prosecutors convince him not to mention it during the trial.
When a string of violent child killings occurs in Derry once again, an adult Mike Hanlon, now the town's librarian and the only one of the Losers to remain in Derry, calls up the six former members of the Losers Club and reminds them of their childhood promise to return should the killings start again. Bill Denbrough is now a successful horror writer living in England with his actress wife, Audra. Beverly Marsh is a fashion designer in Chicago, who has married an abusive man named Tom Rogan, who is similar to her abusive father. Eddie Kaspbrak has moved to New York City, where he runs a limousine rental company and has married a hysterical codependent woman similar to his hypochondriac mother. Richie Tozier lives in Los Angeles and works as a disc jockey. Ben Hanscom is now thin and a successful but lonely architect, living in Nebraska. Stan Uris is a wealthy accountant residing in Atlanta, Georgia, and is married to a teacher named Patty Blum. Prior to Mike's phone calls, all of the Losers had completely forgotten each other and the trauma of their childhood, burying the horror of their encounters with It. However, all but Stan reluctantly agree to return to Derry. After Mike's phone call, Stan is in such fear at the thought of facing It again that he commits suicide by slitting his wrists in the bathtub, writing "IT" on the wall in his own blood. Tom refuses to let Beverly go and tries to beat her, but she lashes out at him before fleeing, causing him serious injury. The five return to Derry with only the dimmest awareness of why they are doing so, remembering only absolute terror and their promise to return.
The Losers meet for lunch in a Chinese restaurant, where Mike reminds them that It awakens once roughly every 27 years for 12–16 months at a time, feeding on children before going into slumber again. The group decides to kill It once and for all. At Mike's suggestion, each person explores different parts of Derry to help restore their memories. While exploring, Eddie, Richie, Beverly, and Ben are faced with manifestations of It (Eddie as Belch Huggins and childhood friends in leper and zombified forms, Richie as a Paul Bunyan statue, Beverly as the witch from Hansel & Gretel in her childhood home, and Ben as Dracula in the Derry Library). Bill finds his childhood bicycle, "Silver," and brings it to Mike's.
Three other people are also converging on the town: Audra, who is worried about Bill; Tom, who plans to kill Beverly; and Henry Bowers, who has escaped from Juniper Hill Mental Asylum with help from It. Mike and Henry have a violent confrontation at the library. Mike is nearly killed, but Henry escapes, severely injured. Henry is driven to the hotel, where It instructs him to kill the rest of the Losers. Henry first attacks Eddie, breaking his arm once again, but in the fight, Henry is killed.
It appears to Tom and orders him to capture Audra. Tom brings Audra to It's lair. Upon seeing It's true form, Audra becomes catatonic, and Tom drops dead in shock. Bill, Ben, Beverly, Richie, and Eddie learn that Mike is near death and realize they are being forced into another confrontation with It. They descend into the sewers and use their strength as a group to "send energy" to a hospitalized Mike, who fights off a nurse that is under the control of It. They reach It's lair and find It has taken the form of a giant spider. Bill and Richie enter It's mind through the Ritual of Chüd, but they get lost in It. In order to distract It and bring Bill and Richie back, Eddie runs towards It and uses his aspirator to spray medicine in It's eye and down It's throat. Although he is successful, It bites off Eddie's arm, and Eddie dies due to blood loss. It runs away to tend to its injuries, but Bill, Richie, and Ben chase after and find that It has laid eggs. Ben stays behind to destroy the eggs, while Bill and Richie head toward their final confrontation with It. Bill fights his way inside It's body and locates It's heart and destroys it. The group meets up to head out of It's lair, and although they try to bring Audra and Eddie's bodies with them, they are forced to leave Eddie behind. They make it to the surface and realize that the scars on their hands from when they were children have disappeared, indicating that their ordeal is finally over.
At the same time, the worst storm in Maine's history sweeps through Derry, and the downtown area collapses. Mike concludes that Derry is finally dying. The Losers return home and gradually begin to forget about It, Derry, and each other. Mike's memory of the events of that summer also begins to fade, as well as any of the records he had written down previously, much to his relief, and he considers starting a new life elsewhere. Ben and Beverly leave together and become a couple, and Richie returns to California. Bill is the last to leave Derry. Before he goes, he takes Audra, still catatonic, for a ride on Silver, which awakens her from her catatonia, and they share a kiss.