It by Stephen King is a novel about friendship and the magic of childhood. The novel centers around seven core characters, friends who bonded as children during a summer of darkness. Twenty-seven years later, they reunite to fulfill a promise made that terrible summer. Evil lives under their hometown and, for some reason they do not understand, these seven have been destined to bring an end to the terrible cycle of death and destruction. These friends fought It once before, but now It is back, and they must end the evil for good. It is a novel of magic, mystery, and horror that will entertain readers of all ages.

Georgie Dunbrough is bored. It has been raining for several days, and Georgie has been stuck inside the house. Georgie's older brother, Bill, is sick in bed, so Georgie tries to entertain himself by making a paper boat. However, Georgie cannot make the boat come out right, so he turns to Bill for help. Bill makes the boat for Georgie and waterproofs it so Georgie can sail his boat in the still flooded gutters outside. Georgie watches the boat sail down the street, excited by this simple game. Then, the boat sails into a drain. When Georgie tries to retrieve the boat, a clown appears in the drain. The clown offers Georgie his boat and, when Georgie tries to take it, the clown tears his arm off, killing him.

Twenty-seven years later, a gay man is beaten to death in the same town of Derry, Maine. Two of the witnesses report seeing a clown assault the man after his attackers throw him into the canal. These reports are buried by the cops investigating, but Mike Hanlon, the head librarian, hears about them and knows what they mean. Mike waits, but after nine children are killed, Mike cannot wait any longer. Mike begins calling his old childhood friends, bringing them back to Derry on the weight of a promise made during the summer of 1958.

Stan Uris, who has become a successful accountant, is the first person Mike calls. Stan does not appear to be upset after hanging up the phone, simply bemused. Stan tells his wife he is going to take a bath. When Stan's wife goes to check on him a short time later, she finds that he has committed suicide. Richie Tozier is a DJ in Los Angeles, a master of many voices. Richie takes the call and is surprised how little he remembers about his childhood. However, Richie begins to remember enough to know returning to Derry, as Mike wants him to, scares him to death. Ben Hanscom walks into a bar in Nebraska and drinks a beer stein of whiskey without showing signs of intoxication. Ben has also gotten a call from Mike asking him back to Derry. Moments after the call, Ben notices scars on his belly he had forgotten he had. Ben is frightened, afraid to go back but aware that he cannot refuse to honor his promise.

Eddie Kaspbrak, a hypochondriac, is packing every drug in his medicine cabinet. Mike's call has left Eddie confused and scared, but he knows he must return to Derry despite his wife's protests. Beverly Rogan is awakened in the middle of the night by Mike's call. Beverly begins to pack while still on the phone, unconsciously lighting a cigarette. Tom, Beverly's husband, is not happy with this as he has told Beverly not to smoke around him. Tom grabs his belt and begins to beat Beverly. For the first time Beverly fights back, allowing Tom the excuse to give a good beating before she escapes the house. Bill Denbrough, Georgie's big brother, is now a popular novelist. Bill is in England when he gets the call. Bill tries to explain things to his wife, but cannot remember enough to explain why he needs to go back to Derry. Bill has begun to stutter, something he has not done since he was a child, and this frightens his wife, Audra.

As each person makes his way back to Derry, they begin to remember little snippets of their childhood. Ben remembers being chased by the town bully into the Barrens, a jungle-like area in the center of Derry. After hiding from the bully, Ben runs into Bill and Eddie. Eddie is having an asthma attack, and Bill needs to go to the pharmacy to get his asthma inhaler. Ben agrees to stay with Eddie while Bill goes. When Bill returns and Eddie has recovered from his attack, they tell Bill how they were trying to build a dam. Ben takes a look at their efforts and offers to show them how to make a better dam. Ben is surprised when they accept his offer and invite him to play with them the following day.

Ben, Eddie and Bill spend the next day building a dam that is highly advanced for their age and experience thanks to Ben's eye for construction. Late in the morning, they are joined by Richie and Stan. While taking a break, Bill tells his friends about a weird experience he had in his brother's room. Bill was looking through a photo album Georgie used to keep. In the album, there is Georgie's last school picture. As Bill is looking at the photo it suddenly winks at him. Eddie tells everyone how he went to the train yards, as he often did on the weekends. When he was walking past a house on Neibolt Street, he saw a leper. A few days later, Eddie returned to the house and went under the porch to peek inside the cellar windows. A leper climbed out of the windows and began to chase him. Eddie knew there was something alien about the leper as he saw orange pom poms, like a clown would wear, on its clothes.

Ben has a similar storyl. Ben was walking home from school late one winter day. When Ben was crossing the canal, he saw the mummy from a movie he had seen recently walking on the ice. The mummy was holding a bouquet of balloons that appeared to float against the wind. Richie denies ever having an experience like these, but Stan begins to tell a story when they are interrupted by Mr. Nell, a local beat cop. The cop tells them that because of the complicated drainage system in Derry, the dam that the children have built causes pipes to backup all over town. The children are told to dismantle the dam.

Richie talks Bill into showing him Georgie's picture album. They go to Bill's house and sit alone in the room with the album between them. The school picture of George is gone and in its place is an old photo of downtown Derry. As Richie and Bill watch, the picture begins to move. Bill is so fascinated by this phenomenon, he tries to touch the photo. Instead of touching the surface of something solid, Bill's fingers move into the picture. Richie pulls them out only to discover that Bill's fingers are cut where they entered the picture.

A few days later, Richie goes to the movies with Beverly and Ben. They watch a movie about a teenager who turns into a werewolf and one about a teenage Frankenstein. During the first movie, they see Henry Bowers, the town bully, and his friends below them. Ben is worried as Henry is angry at him for escaping into the Barrens the last day of school. Richie tells him not to worry. When the movies are over, they wait for Henry and his friends to leave. Richie leads Ben and Beverly out through a side door. However, Henry and his friends are waiting for them in the alley. Henry tries to beat up Ben, but for the first time, Ben and his new friends are able to get the better of Henry and escape.

Days later, Bill tells Richie he wants to investigate the house on Neibolt Street. Bill takes his dad's gun in case there is any trouble. Bill and Richie crawl under the porch and enter the cellar through the same window where Eddie said the leper come out. Once inside the cellar, Richie sees a werewolf come down the stairs. The werewolf chases the boys into the coal chute where Bill lifts Richie up to try to escape through the window. As Richie is trying to get the window open, he can hear Bill screaming at the werewolf and firing his father's gun. Richie climbs out the window and reaches back to help Bill. Just as Bill is about to come out the window, the werewolf grabs his leg. Richie manages to get Bill out, but the werewolf is not far behind. Richie blows sneezing power at the werewolf, which gives him and Bill enough time to reach Silver, Bill's bike. The werewolf comes after Bill and Richie as Bill attempts to get the enormous bike moving. The werewolf manages to grab Richie once, but somehow the two boys make it to a populated area without being injured.

The day after seeing the movies with Richie and Ben, Beverly is in her bathroom preparing for bed when she hears voices coming from the sink drain. Beverly bends over and speaks to the voice, thinking it must be a child playing in the building's basement. Suddenly, blood erupts from the drain, splattering the bathroom. Beverly screams, bringing her father into the room. However, Beverly's father cannot see the blood. The next morning, the blood is still there, but Beverly's mother cannot see it either. When both her parents have gone to work, Beverly goes and finds Richie, Ben and Stan. Beverly takes them to her apartment and shows them the blood. They can see it. Richie, Ben and Stan help Beverly clean up the bathroom. Afterward, Richie, Ben, Stan and Beverly go to a Laundromat to clean the rags. While they are waiting, Stan tells them how he was bird watching in Memorial Park and was nearly attacked by two dead teenagers in the Standpipe.

When everyone has arrived in Derry, Mike arranges a reunion lunch. During lunch, everyone catches up on their lives. Afterward, Mike tells them about the murders and his reasons for calling. One of those reasons is a message found near the body of the last victim, the words come home written over and over in the victim's blood. Everyone agrees that it has begun again even though they cannot really remember what it might be. The waitress comes in with fortune cookies for dessert. When they begin to crack open the cookies, strange things come out. Ben's has teeth, Eddie's a cricket, Richie's an eyeball. However, they are the only ones who can see these things.

After lunch, Mike suggests that everyone take a walk around Derry alone and then meet at the library after closing for another discussion. Ben goes to the library during his walk, a place where he spent a great deal of time as a child, and runs into the clown, Pennywise, there. Eddie goes to the field where a group of kids always played ball when he was a child. Eddie is nearly killed by the walking corpse of an old acquaintance. Richie walks downtown to Bassey Park and is pounced upon by the giant statue of Paul Bunyan. Beverly returns to the apartment where she and her parents lived only to find Pennywise waiting for her in the form of an old woman. Finally, Bill takes a walk, talking to a couple of kids, and happens upon a second hand store where he finds Silver for sale in the window.

That night when they meet in the library, each person begins to tell stories about the things they have begun to remember from that summer in 1958. Mike recalls the rock fight that brought him into the circle of friends. Mike was on his way to the church where he attended school when Henry Bowers and his friends began to chase him. Mike ran into the train yards near the town dump. Bill and the others were walking to the dump from the Barrens side and ran into Mike. Bill told everyone to gather rocks just as Henry Bowers and his friends appeared. Bill begin to throw rocks and, before anyone realized it, Henry and his friends were beat back. When it was over and Henry was gone, promising revenge, Bill realized that Mike was the last piece of their puzzle of friends. The Losers Club was now complete.

A few days later, the Losers began planning and building a clubhouse in the Barrens. They had wanted to build a treehouse, but Ben came up with the idea of digging a clubhouse out of the soft ground, creating a place hidden from outsiders. While they were building this clubhouse, Mike brought his father's album of historic pictures to the Barrens to show everyone. In it were many pictures of the clown they had all seen, at various times stretching over a hundred years. While they were looking at these pictures, one of them began to move much like the picture in Georgie's album had done.

Shortly after the clubhouse is finished, Ben tells everyone about a tradition he read about in which Indians would use a den such as their clubhouse and burn green wood in order to induce a vision. Bill decides that they should try this. Bill also decides that Beverly should remain above ground in order to help anyone who is overcome by the smoke. However, something else does not want anyone to remain outside. When Beverly insists they draw matches, the burnt match that is supposed to decide the loser somehow disappears. All seven go into the clubhouse and sit in the smoke until they cannot stand it any longer. Mike and Richie stay in the longest and have a vision in which they see the day that It came to Derry. It is a time long ago, perhaps the beginning of time, and It comes from the sky in something like a spaceship.

Bill decides they should do research on these phenomenon and try to find a way to kill It. Bill believes that a silver bullet can stop the creature, but does not know how to make one. Bill gets Ben to help. They decide that rather than make a silver bullet, they should make silver slugs that can be shot with a slingshot. Beverly is chosen to shoot the slingshot, because she is the best of them all. Everyone goes to Bill's house one night and watches as Ben makes the silver slugs. A few days later, the Losers go back to the house on Neibolt Street to confront It. There they manage to make the creature emerge from the pipes in the bathroom. It attacks Ben, nearly disemboweling him. Beverly manages to get a shot off and injures It, causing it to run away.

The Losers know that they will have to confront It someday, but are not sure when or how. The decision is made for them in August when Henry Bowers comes after them once again. Bill has Ben lead them to a pumping-station where they can enter the tunnels that comprise the drainage system that runs under Derry. Once inside, Eddie is given the task of leading them to Its nest. Bill does not know what they will do once they arrive, but in his research, he came across a ritual called Chud. In this ritual, a person is supposed to bite into the tongue of the glamour, the shape changing creature, and tell jokes. The first one to laugh is killed by the other. When they reach Its nest, Bill does this, only with his mind rather than his body. Instead of telling jokes, Bill stands up to the creature, proving his lack of fear. This show of fearlessness causes It great injury, and it sneaks away to a deeper part of Its lair. Most of the others believe it is dead, but Bill has his doubts. When they leave the tunnels, Stan cuts each of their hands, and they make a blood oath to return if the killing starts again.

In the present time, Mike sends everyone to bed, because he knows they will have to reenter the tunnels very soon. While Mike is cleaning up, Henry Bowers enters the library and attacks him with a knife. Mike is mortally wounded, but manages to wound Bowers, as well. Bowers leaves the library and goes to the hotel where the Losers are staying. Bowers goes to Eddie's room and attacks him. Eddie manages to fight back and gets the better of Bowers, stabbing him in the belly with a broken Perrier bottle. Eddie calls Bill, and he makes the decision that they must return to the tunnels immediately.

Once in the tunnels, Bill realizes that It has somehow brought his wife there. Bill is upset, but manages to focus on the task ahead. Once in Its nest, Bill again begins the ritual of Chud. However, Bill is not strong enough alone. Richie joins in and then Eddie. Eddie attacks It and sprays It with his aspirator. It bites Eddie's arm off, but not before it is badly wounded from Eddie's aspirator. It retreats once again, but this time Ben, Bill and Richie follow. Ben sees that It has laid eggs and stops to kill them while Bill and Richie go after It. Bill and Richie attack It, and Bill manages to hit Its heart.

When everything is over, all six begin to lose their memories of the event, including Mike. Richie, Ben and Beverly return home. Bill stays in Derry with his wife who is in a catatonic state as a result of seeing It. Bill gets the strange idea to take her for a ride on Silver. Somehow, this ride brings Audra out of her catatonic state, returning her to the same healthy woman she was before.