1957–58
In October 1957, the town of Derry, Maine, has been flooded by autumn storms, awakening an evil entity, known only as "It", a shape-shifting monster that targets children, and lives in the sewers. As the storm abates, six-year old George "Georgie" Denbrough chases a paper boat, made with the help of his brother William (also known as Big Bill, Stuttering Bill, or simply Bill), along a gutter current until the boat is swept into a storm drain. Georgie attempts to retrieve the boat and encounters Pennywise, a seemingly friendly clown standing in the sewer. The clown entices George with sensations of a circus carried into the sewer by the floods, and offers to return the lost boat. When George attempts to retrieve the boat, Pennywise rips his arm off, killing him instantly. When the neighbors hear George's scream, they run outside to see what the matter is, but only find a one-armed George, with Pennywise nowhere to be found.
Eight months later in June 1958, on the last day of school, Ben Hanscom flees from local bullies Henry Bowers, Belch Huggins, and Victor Criss. Ben escapes into the Barrens, a weedy jungle through which runs the Kenduskeag Stream, where he meets and befriends Bill Denbrough (George's brother) and Eddie Kaspbrak. As a summer project, Bill and Eddie were attempting unsuccessfully to dam the Kenduskeag. Ben has a natural inclination toward architecture, and helps the other boys complete the dam. Through Bill and Eddie, Ben also befriends Richie Tozier, a wisecracking boy known for his dubious "Voices", and Stan Uris, a straight-laced Jewish boyscout. In early July, the five of them, along with their new friend Beverly Marsh, save Mike Hanlon, the only black child in Derry, from being beaten by Henry Bowers and his gang. A few days after fending off the bullies, Mike officially joins their group. The children establish themselves as the "Losers Club." All are outcasts, emphasized by their unhappy home lives and the torments they have endured from bullies like the Bowers gang: Ben because of his weight, Bill because of his stutter, Eddie because of his physical frailty, Richie because of his smart mouth and thick glasses, Stan because of his Jewish ancestry, Mike because he is black, and Beverly because of her poverty and gender. As their friendships mature, they realize that they also share common encounters with It.
Bill describes to the group how, while lamenting his dead brother George in his room, he had opened the boy's photo album to see a picture of George come alive. Ben reveals that he had encountered a Mummy the previous January, while walking home from school. Richie eluded Henry and his friends in town and, when he sat down beside a statue of Paul Bunyan it became possessed and attacked him, almost killing him. Richie is unique amongst the group because he doesn't tell his account to the others and firmly believes it to have been a dream. Eddie recounts being attacked by a diseased creature which he calls a leper, at an abandoned house. Beverly tells of hearing voices of dead children coming from her bathroom sink drain, followed by a gruesome gout of blood bursting from the pipes; her father and mother see nothing. In an early depiction of the Losers' strength when united, Beverly is unable to clean the blood herself until she enlists the help of Ben, Eddie, and Stan. After cleaning up the blood, Stan describes his encounter with the water-logged corpses of children when he became trapped in the town's Standpipe while birdwatching, which he somehow fends off by shouting out the names of birds. Richie initially scoffs at the stories, and Bill encourages him to view the photo album back in George's room. After both boys discover in the album pictures of Pennywise, one of which comes to life, Richie recalls his own encounter with a Werewolf. Determined to avenge his brother, Bill steals his father's Walther handgun and goes with Richie to the abandoned house, 29 Neibolt Street. At the house the boys are attacked by It, each perceiving a different form of the creature: Bill sees It as Pennywise and Richie sees It as a Werewolf. Both barely escape with their lives on Bill’s enormous and talismanic Schwinn bicycle, Silver. Mike then retells his encounter with an enormous bird, which he had fended off by throwing a chunk of tile in its eye.
The Losers are not the only persons to encounter It; various children vanish to be found dead and mutilated days, weeks, or months later, if they turn up at all. The Losers determine to destroy the supernatural being and seek out means to that end. After some research in the town library, Bill discovers an ancient spell known as the Ritual of Chüd, in which a shapeshifting monster called a "talus" and a human shaman lock tongues and tell jokes; the first to laugh is devoured by the other. Bill believes this ritual will allow them to defeat and kill It. While the seven are building an underground clubhouse in the Barrens in mid-July, Mike Hanlon brought his father's photo album and shows it to them all. The group discovers that Pennywise has existed for at least centuries in the Derry area. In one photograph, Pennywise again appears and threatens to kill them all, appearing to them all as their worst fears. The Losers are demoralized and uncertain how to proceed. Ben gives them the idea to perform an Indian "smoke-hole" ritual to receive spiritual guidance from whatever forces stand against "It". They use their now completed underground club house and green wood to cause a blinding smoke, forcing most of the Losers into fresh air. Richie and Mike are the only two to withstand the smoke, and witness a vision of It arriving on Earth in prehistoric times. When the visions end, the Losers express still further doubt over their ability to battle the monster.
A few days after the smoke-hole ritual, Eddie goes to a pharmacy to pick up his asthma medicine. The pharmacist reveals that the medicine is actually a placebo, and Eddie is not actually sick; his "asthma" is an invention of Eddie's mother Sonya Kaspbrak. Eddie leaves disbelieving, and, on the way home, is attacked by Henry, Victor, Belch, and a psychopathic boy named Patrick Hockstetter. The bullies break Eddie's arm and spit in his face in retaliation for the rock fight. Eddie convalesces in the hospital, and though the other Losers attempt to visit him, they are sent away by Eddie's overprotective mother. Mrs. Kaspbrak is cast as an unwitting agent of It, the creature attempting to use her maternal concern to split the Losers. Eddie stands up to his mother for the first time, and earns a small amount of autonomy from her ministrations.
As Eddie recovers, Beverly stumbles across the Bowers gang (including Patrick) in the landfill, literally with their pants down, lighting farts for fun. She hides behind a junked car, afraid they will see and attack her, and waits for the gang to depart. After Belch and Victor leave, Patrick (who, in a back-story, is revealed to have murdered his baby brother) masturbates Henry, and offers him oral sex. In response, Henry threatens to reveal Patrick's secret: besides killing his brother (of which Henry knows nothing), Patrick has been trapping animals in an abandoned refrigerator and leaving them to suffocate. After Henry leaves, Patrick decides to dispose of the animal corpses, but when he opens the refrigerator, he is attacked by It in the form of dozens of winged leeches, his worst fear. Before he loses consciousness, It appears in the form of a man with a melting face and drags him to It's lair. Patrick wakes up as It begins to feed on him. After Beverly's escape, the Losers (sans Eddie) return to the refrigerator and discover a message from It written in Patrick's blood, warning them to stop before It kills them, which sends Bill into a fit of maniacal rage. At Bill's pleading, the others agree to help him.
After Eddie is released from the hospital, Ben makes two slugs out of silver, believing the cinematic convention that silver will kill monsters. The group test fires slingshots and determine Beverly to be the best shot, and so the slugs are put into her care. The Losers return to 29 Neibolt Street and enter the house, its interior made magically huge and vertiginous by It. In a run-down bathroom It attacks the Losers in its Werewolf form, primarily focusing its efforts on Bill, hoping to destroy the head of the Losers. After It injures Ben, Beverly injures It with one slug, but loses the other; however, the Losers chase It away by convincing It that a final slug is ready to be released upon it. The Losers realize that their united belief is the strongest weapon against It.
In August, It turns to Henry Bowers, whose sanity had been eroding throughout the summer, as It's minion. It provides Henry with a switchblade, with which the boy promptly murders his crazy, abusive father. Henry recruits the unsuspecting Victor and Belch and takes them into the Barrens, where they drive the Losers into the sewers. Under Derry, It attacks the Bowers gang in the form of Frankenstein's monster, decapitating Victor and ripping half of Belch's face off, though Henry escapes. Wandering aimlessly for hours in the sewers, Henry ultimately falls into the Canal and drifts back into the Kenduskeag, where he falls unconscious for several hours. When he comes to, Henry returns home to find the police there waiting for him; as it turns out, It has framed Henry for most of the people that It killed both before and during the summer of 1958 (including Henry, Victor, and Belch).
The Losers press deeper into the sewers and confront It in the form of a giant Eye, which Eddie defeats using his asthma inhaler, believing it to be battery acid. Finally the Losers enter Its lair, a chamber deep below the heart of Derry. The creature appears as a giant Spider. Bill enters Its mind through the Ritual of Chüd and comes to a darkness beyond the universe, where Its true form resides. With the help of a galaxy-spawning entity called the Turtle, Bill defeats It and the monster retreats, grievously wounded. The Losers decide that It has been destroyed and attempt to leave the sewers, but find themselves lost. As the Losers panic in the dark, their mystical bond begins to fray. In order to restore the bond so that they may return to the surface, Beverly has sexual intercourse with each of the boys.
The Losers finally escape from the sewers, emerging at sunset. Stan cuts their palms with a shard of a Coke bottle and the seven make a blood oath to return to Derry if It ever resurfaces.
1984–85
In July 1984, two homosexual youths named Adrian Mellon and Don Hagarty are assaulted and Adrian is thrown off a bridge by three homophobic bullies. They are arrested for murder when Adrian's mutilated corpse is found, though one of the murderers (as well as Mellon’s own boyfriend) claims that he saw a clown kill Adrian underneath the bridge. When a string of violent child-killings hits Derry following Adrian's death, Mike, now the town’s librarian and the only one of the Losers’ Club to remain in Derry, calls up his six friends on the night of May 28, 1985 and reminds them of their childhood promise to return. Bill is now a well-known writer of horror novels (after King's own self), living in England and married to an actress called Audra; Beverly is in the fashion industry and very successful but married to an abusive man called Tom Rogan; Eddie runs a limousine rental company and is married to a woman exactly like his mother named Myra; Richie is a professional comedian with a radio show and talk show; Ben is now thin and a successful architect; Stan is an accountant married to a woman called Patty. Five of them return to Derry with only the dimmest awareness of why they are doing so, having almost completely blocked out virtually every aspect of their childhood (most notably their encounter with It). Stan, who it is implied still remembers the entire thing (or at least does when Mike contacts him) commits suicide in fear and dread of another encounter with It by slitting his wrists while in the bathtub, and writing IT on the wall with his dying strength.
The remaining Losers’ Club meets at a Chinese restaurant for lunch on May 30, where, after a long meal and stories from the intervening years, Mike enlightens them to the apparent nature of It, gleaned from a large amount of research he has done himself: It awakens once roughly every twenty-seven years for twelve to sixteen months at a time to feed on children before going into slumber again. He suggests that, due directly to their intervention in the summer of 1958, they injured It so badly that the cycle, which usually came to an end in the winter months of the year, stopped abruptly and prematurely in August. The group holds a vote in which they decide to kill It once and for all. Mike suggests that before deciding what exactly to do, each Loser takes a walk around Derry to become re-acquainted with their childhood home and meet up at the library later, and also decides not to tell them anymore and allow them to remember what occurred on their own, fearing that they may commit suicide as Stan had done if they discover what happened before they are ready to remember it all. As they finish the meal, their fortune cookies are revealed to have a multitude of disgusting things inside of them, such as a huge cricket and a human eye, courtesy of It.
While walking around Derry, many of the Losers witness manifestations of It. Ben goes to the library, his favorite place in Derry as a child, and sees It, both as Pennywise and as Count Dracula, who warns him to leave the city by nightfall. Eddie goes to an old baseball field and is attacked by It in the form of the leper that attacked him on Neibolt Street and as zombies of several of It's 1958 victims. Beverly goes back to her father's house and is greeted warmly by a kindly old woman named Mrs. Kersh who turns out to be the daughter of IT, who first appears in her true form of a witch of the sort from the tale of Hansel and Gretel, and then IT attacks first in the form of her father, who It claims died five years before, and then as Pennywise. Richie goes to a statue of Paul Bunyan and It appears to him there after Richie recalls that It tried to kill him in the form of the statue. The four all escape danger. Bill, while not seeing It, meets a kid who challenges him to ride his skateboard. Bill goes to an abandoned second-hand shop, and also finds his old childhood bike there, remembering how it saved his and Richie’s lives. He purchases it and takes it to Mike’s home to fix it up.
Unknown to the Losers, three other people are also converging on Derry: Bill's worried wife, Audra Phillips; Beverly's abusive husband, Tom Rogan; and Henry Bowers, who is driven by It to escape the mental institution where he resides and get revenge on the Losers.
The Losers meet at Mike’s library after closing time and reminisce about the summer of 1958. Afterwards, the Losers leave for their hotel rooms. Mike stays at the Library a little longer and is confronted by Henry. After Mike informs Henry that It will most likely kill him after he kills the Losers, they fight and Henry stabs him in the leg, hitting his femoral artery. Mike badly injures Henry with a letter opener, but Henry is able to escape, and Mike, using his belt as a tourniquet, calls the hospital and successfully gets help, despite Pennywise's attempts to block him. Henry, with the guidance of It (in the form of Belch’s reanimated corpse), and transported by the cursed car Christine, goes to the hotel and attacks Eddie. Henry successfully breaks in to Eddie’s room, but Eddie manages to disarm Henry, gouge out his right eye with a broken bottle, then stab him in the hand. Henry knocks Eddie back and Eddie breaks his arm in the same place that Henry had broken it before. Henry then falls on him and is impaled on the bottle, finally dying.
Meanwhile, It appears to Tom and orders him to capture Audra. Tom brings Audra to It's lair under the city. Upon perceiving It in true form, "the deadlights" (bright orange lights that can hypnotize, and even kill people, if exposed to them long enough) Audra becomes catatonic and Tom drops dead in shock. Bill, Ben, Beverly, Richie, and Eddie, after calling the library and finding out that Mike may be near death and understanding that the town, which is essentially under the control of It, will not help them, realize that they are being forced into another confrontation with It. They descend into the sewers.
While in the sewers, the remaining Losers 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. Later, deep within the sewers, It appears as George but Bill overcomes the illusion. They reach It’s lair again. Bill and Richie engage It in the Ritual of Chüd again, but Bill realizes that "the Turtle cannot help them" due to It killing the Turtle by feeding the Turtle galaxies and letting it choke to death. Richie rescues Bill from the deadlights and manages to severely injure It. Eddie helps them and saves their lives, but he is killed in the process. Beverly stays with Eddie and the traumatized Audra, who has been woven into a giant spider web by It’s Spider form. Ben starts destroying the eggs that It had lain. Bill and Richie follow It deeper into the cavern and attack It. Bill crushes Its heart between his hands, finally killing It. At the same time a storm sweeps through Derry and the downtown area collapses. Later, Michael, writing in a journal, concludes that Derry is finally dying.
The novel ends with the various Losers returning home and forgetting about It, Derry and each other all over again. As a sign that It really is dead and a watchman is no longer needed, Mike’s memory of the events of the book also begin to fade, much to his relief. Bill is the last to leave Derry. Before he goes, he takes Audra, who is still catatonic, for a ride on Silver, hoping that they can beat her catatonia the same way he and Richie beat It in 1958. They succeed, and the story concludes with Bill musing over his forgotten childhood.