Bill Densbrough arrives in town and checks in at the Derry Inn Hotel. After talking with the front desk clerk, a fan of his horror novels, he takes a taxi to the cemetery to visit his brother Georgie's grave. As he looks down at it, he apologizes for forgetting him since he left Derry. The sound of someone digging reaches Bill's ears and he looks up to see Pennywise digging the last of seven graves. Pennywise offers to let him choose anyone he wants except for the last one, which is already filled in. Bill tells It that he remembers that they beat It before and they'll do it again. He insists that he's not afraid, and Pennywise and the graves disappear.
At the library, Bill discovers that Mike has set up a tribute to him as Derry's resident writer. He finds Mike and is momentarily shocked at how much older Mike looks than his 42 years. They embrace and the two men go outside. When Bill says that he still doesn't remember much of what happened in 1960, Mike explains that he never forgot because he never left Derry like the others. They walk down the street and Bill realizes that they're in what was the poor part of town 30 years ago. Mike says that it still is and walks over to his nearby house.
Inside, the two men drink beer and Mike says that he's made reservations for dinner that night for the entire group. Both of them admit that they can sense the others, and they're sure that their friends haven't arrived in town yet. Mike then takes Bill out to his garage and shows him his old bike, silver. He explains that he saw it a year ago in a pawn shop and bought it. However, Mike tells Bill that three weeks earlier he bought a tire-repair kit on an impulse, and wonders if the same force that guided them in 1960 is working now. Bill touches his old bike and remembers back to 1960 as he rode away with Stan on the back, escaping from something that was chasing Stan.
Richie arrives in Derry and discovers that they've closed the Paramount Theater. As he parks next to it, he sees the marquis sign display his years of birth and death, the later in 1990. Next Richie drives to the library and asks for Mike, and the assistant says that he'll be back in a while. Richie sits down and tries to read a magazine, but Pennywise calls down to him from the balcony. None of the library patrons can hear Pennywise, who taunts Richie and offers him a balloon. Richie starts to answer but realizes that nobody else can hear Pennywise. When Richie stares in horror, hundreds of balloons fall down from the balcony, popping and covering everyone in blood that only Richie can see. As Pennywise screams jokes, Richie tries to make himself heard and all but yells at the library aide, telling her to tell Mike that he'll see him tonight. Once he does, he runs out and drives away.
Mike comes out as Bill finishes repairing Silver, and the two of them ride around the nearby playground. Mike then takes out a deck of Bicycle playing cards to pin to the spokes. As Bill takes them out, he drops the cards and they all land face down except for two cards: two identical Ace of Spades. All of the other card backs have a picture of Pennywise on them.
Ben arrives in town by taxi and goes past the Barrens. He tells the driver to pull over and goes down the slope that he and the other Losers dammed up as children. Bullies chase a boy who looks like a young Ben down to the creek, just as Henry Bowers and his gang chased Ben 30 years ago. They shove the boy to the ground and Ben tends to his scraped knee. After the boy leaves, Ben hears his father's voice call to him. He goes to the lake and sees the skeleton of his father standing on the lake. Ben insists that it isn't real and concentrates, and the skeleton disappears. As he goes back to the creek, a bum comes up behind him, startling him. Ben climbs back up to the taxi and tells the driver to continue into Derry, and he sees Pennywise waving to him from the side of the road. A balloon appears next to Ben with the words "Turn back now" on it.
Eddie arrives in Derry and goes to the drugstore to refill his medicine. As he goes into the store, he remembers how in 1960, the owner, Mr. Keene, called him into his office and told him that his family doctor was prescribing a placebo to make Mrs. Kaspbrak happy. Eddie picks up his prescription from Mr. Keene's grandson and hears Mr. Keene call out, asking his grandson for a cigar. Eddie goes back to see the much older Mr. Keene, who appears to have no idea what is going on around him. Eddie thanks him for telling him the truth about the placebo, even if it didn't do any good, and Mr. Keene appears to recognize him. The old man grabs Eddie's wrist and speaks in Pennywise's voice, telling Eddie to get out of Derry while he still can. Eddie pulls free and runs out.
When Beverly arrives in Derry, she goes to her old home to see her father. She steps up to the door and sees the name Marsh on the doorbell. She rings it but an old woman Beverly doesn't recognize comes to the door. She introduces herself as Mrs. Kersh, and Beverly looks at the doorbell, which now says "Kersh." She apologizes for the mistake, and Mrs. Kersh invites her in for tea. Beverly goes to the bathroom and freshens up, and runs a little water in the sink. She blacks out and only wakes up when Mrs. Kersh calls to her. Coming out, Beverly sips at her tea while Mrs. Kersh slurps down her entire cup in a single gulp. As Beverly looks on in horror, Mrs. Kersh transforms, her teeth rotting and her face melting. Looking at her cup, Beverly sees that her tea has become blood. Realizing that the doorbell label did say "Marsh," she looks up at Mrs. Kersh, who has partially transformed into the image of her father Al. He tells Beverly to get out while she can, and Beverly runs for the door as her father chases after him. She runs out into the street and turns to see Pennywise on the porch, laughing. Beverly narrowly avoids a truck and turns to see that Pennywise has disappeared and that the house has transformed into an abandoned wreck. A balloon appears next to her and floats down the street.
In England, Audra talks to the film producer, Greco. She tries to tell him why Bill had to leave and suggest that she might go after her husband. Greco warns that they can get by without Bill, but if she abandoned the production then she'll never work in film again. He tells her that she'd be best off following Bill's request not to follow her. Audra considers it but once he leaves, calls the airplane and books a flight to the U.S.
Eddie arrives at the Chinese restaurant that Mike has booked for them and runs into Ben first. They hug as Mike and Bill arrive. Richie comes in next, sees the others, and goes into the restroom to get a grip. Once he does, he comes out and does his usual wisecracking act. Beverly arrives last of the six and stare at them all together, and then collapses. They wake her up and she explains that she had a sudden rush of memories when she saw them. She wonders what they're getting into and then hugs Ben... and kisses Bill. They compare notes and realize that Pennywise warned all of them to get out of town. The Losers wonder if Pennywise is afraid of them, and remember their confrontation with It in the sewers. Eddie suddenly gasps and has an asthma attack as he remembers using his asthma inhaler on Pennywise, pretending it was acid. He apologizes for his outburst and Bill reminds him that he saved Stan's life. They wonder where Stan is and Mike says that their seventh member will be there soon.
Bill immediately wants to come up with a plan to confront It, but Richie says that they have time and he wants to relax and learn what everyone has been doing. They sit down for dinner and share a toast to the Losers Club. As they talk, they realize that none of them have children. Eddie claims that he has settled down with a beautiful woman. When they remember Henry Bower, Mike tells them that Henry has been locked up at the Juniper Hills Psychiatric Prison ever since he confessed to the murders in 1960. They remember that they kept silent when Henry took the blame, even they knew that It was responsible. When Bill insists that they stopped it, Mike warns that they didn't.
At Juniper Hills, Henry is in his bunk sleeping when he hears Pennywise calling to him from the Moon. He looks down and tells Henry that the Losers are back and that Henry can get revenge on them.
As the Losers finish dinner and their waitress, Rose, brings the fortune cookies. Richie says that it's been fun but he's going back to Los Angeles. Eddie agrees, but Ben and Bill say that they need to finish the job. Mike tells Richie and Eddie that they kept their promise and as far as he's concerned, everything is square. Mike insists on paying for the dinner and they open their fortune cookies. Something they fear appears out of each of their fortune cookies, but they realize that no one else can see the manifestations. They assure Rose that everything is fine and quickly run out.
The Losers retire to the library and Mike calls Stan in Atlanta to see where he is. As they see the blood from the balloons, Beverly remembers the blood in her bathroom. Mike finally reaches Stan's wife and learns that he committed suicide in his bathtub. Bill remembers that Stan was the last of them to see Pennywise for the first time in 1960. Stan had fled from It and Bill picked him up and rode off to safety on Silver. Once they met the others, Stan told them how he was bird watching when he heard a voice calling to him from a nearby abandoned house. Going in, he saw It coming down the stairs in the form of a mummy. Using his bird watching guide as a token of faith, Stan recited the names of birds, holding off It until he could run out and find Bill.
Eddie remembers that Stan saw the real It on the day of the confrontation, and looked into its "deadlights." Mike goes to the library refrigerator to get some more beers and finds an illusion of Stan's severed head on a shelf. Balloons pour out and "Stan" tells them that he's in the deadlights and soon they will be as well. Mike slams the refrigerator door shut but a wind springs up out of nowhere and it begins raining indoors. As books fly off of the shelves, the six friends join hands and form a circle, and Its power fades away. As they clean up, Bill discovers that the typewriter typed on its own, spelling out the message, "He thrusts his fists against the posts, and still insists he sees the ghost." Ben reads it and Bill explains that it was an exercise that his mother gave to him to help with his stutter. They grab the equipment that Mike has bought to explore the sewers and leave for the Derry Inn.
At Juniper Hills, It takes the form of Belch and appears to Henry. The guard, Koontz, hears Henry seemingly talking to himself and tells him to shut up or he'll beat him. Belch tells Henry to think to him instead of talk, and then explains that the Losers have returned to Derry and they can both get revenge. To do so, Henry has to escape from the prison. When Henry wonders why It needs his help, It explains that it can kill them if they half-believe, but Henry can kill them whether they believe or not. Henry says that he can't get past Koontz, and "Belch" gives him his old switchblade knife and says he'll take care of Koontz. Henry starts walking toward Koontz, and the eagerly guard prepares to beat him. Pennywise steps out from behind Henry, taking the form of a Doberman Pinscher, Koontz's greatest fear, and the guard screams in terror as It kills him.
At the hotel, the Losers meet in one of the rooms and unpack Mike's hardhats and flashlights that he's bought for the expedition. Richie insists that he isn't going, while Eddie has another asthma attack. He takes his asthma medicine and admits that he knows that it's a placebo, and Beverly assures him that as long as he believes, its good medicine. Mike then hands out clippings of the history of Derry that he's gathered over the years. Every 30 years there has been a series of child deaths and disappearances, and then the series ends in a disaster. The adults don't want to know what is happening and deliberately blank out anything evil that they see. Beverly remembers a neighbor man ignoring her when Henry and his gang tried to attack her, and it was only because of her father that the bullies fled.
Mike explains that now that it's 1990, a new series of killings have started, and he found Georgie's photo at the scene of the most recent death. He shows the Losers the photo, and says that the same thing that protected them in 1960 is bring them together now. That same force has made all of them successful, but kept them from having children. When the others point out that Mike was left behind and doomed to life a life of poverty, he says that none of them had any choice in the matter and it was their parents who chose to leave or stay. Once he grew up, he decided to stay on his own so that he could serve as the "lighthouse keeper" and watch for Its return. The others thank him for everything that he's sacrificed.
Audra drives into Derry and stops at a gas station to ask for directions. The gas station attendant is Pennywise in another guise. He directs her into town and offers her a balloon, and Audra tries to drive away. Pennywise's eyes glow with the deadlights, entrancing her.
Ben talks about how he lost weight in high school after the coach insulted him. Mike asks what they're going to do next, but Richie says that he's leaving come sunrise. Beverly remembers calling the Losers to her house to see if they could see the blood from the sink. They could, confirming that she wasn't insane, and helped her clean up the blood. Once they worked together, the blood never came back. Ben offers to get Beverly's shawl from her bedroom, and Bill says they should try and stay together. Mike and Eddie go with Ben to the top floor and they go to their separate rooms, leaving the doors open. Ben goes to Beverly's room and gets the shawl. In his room, Mike notices that the window is open and closes it. Henry comes out of the bathroom and stabs him in the chest with his switchblade.
Ben looks up as Beverly enters the room and recites the haiku he wrote for her as a child. He finishes it and she says that she knew that he wrote it. Beverly kisses him and Ben wonders if she ever thought Bill had written it. She insists that she knew it came from Ben, and that she has been waiting until they were both ready. As they kiss, Ben glances in the mirror behind Beverly and realizes that she's wearing clown pants, and his face is covered in clown makeup. He runs for the door, turns, and sees that "Beverly" is really Pennywise. The clown disappears and Ben desperately wipes the clown makeup off his face.
Henry tries to finish the job with Mike, driving the knife down toward his chest. Ben and Eddie hear the struggle and break in. They grab Henry, who stabs himself in the chest during the struggle and dies. Ben and Eddie call the others and they all drive Mike to the hospital.
The next morning, the five remaining Losers wait for Mike to come out of ICU. Ben and Beverly wait outside, and she tells him that she ended up dating a man who was just like her father. When she wonders how she could be so blind, Ben tells her that at least she finally realized the truth, and she fell in love with someone. Beverly recites the haiku and realizes that Ben wrote it, but Ben backs away, wondering if it's her or Pennywise. Beverly insists that it's really her and finishes the haiku. They start to kiss, but Eddie comes out and calls them in. The doctor tells them that they've stabilized Mike and they can talk to him for a minute.
Bill goes in alone and tells Mike that the police haven't been called in, and if they do then the Losers will claims that Mike got drunk and injured himself. Mike shows Bill the two silver earrings that they used against It 30 years ago, He tells Bill that ten years ago, he was suffering depression and went into the sewers to find the earrings, half-hoping that It would find and kill him. His hair turned white but he made it out alive and since then, he got his life back together. Mike tells Bill that he'll understand if they would rather leave than use the earrings against It.
Back at the hotel, the Losers cover Henry over. When they wonder if they should go to the police, Bill warns that the entire town of Derry is in some way It. If they get the townspeople involved, something bad will happen to them. Richie and Eddie still want to leave town, but as Richie turns off the lights, he hears a TV report about the death of another girl. As they go out through the lobby, Bill hears a woman playing the piano in the lobby. He looks in and sees the woman playing with a young boy, just like his mother did when he was a child.
Bill goes out to the drive and tells the others that he just saw something and doesn't know if it's real or an illusion, but he can't live being unable to tell the difference between the two. He tells them that he's going to kill It once and for all, for Georgie and for all the other children. The others embrace him while Richie looks on.
The five remaining Losers go to the Barrens, and Beverly shows them the slingshot that was the only thing she took away from Derry. They remember that she cracked open Pennywise's head, revealing the deadlights within. Bill shows them the earrings and relates how Mike recovered them. They all prepare to go except Richie, who insists that they're all going to die. When Bill says that he can leave if he wants, Richie admits that he couldn't leave any more than he could resist coming.
The group enters the sewer plant and follows the same trail that they used 30 years ago. They find a purse and Bill recognizes it as Audra's. Panicking, he runs ahead on his own, trips, and cowers in the shadows. The others find him and insist that it's not his fault, and that they need to face It and kill it, or it will pick them off one at a time. After a moment's hesitation, Bill gets control of himself and they proceed.
Eddie directs them to the central sewer juncture where they faced It 30 years ago. Georgie's newspaper boat comes down a floor channel and stops at Bill's feet. Georgie appears and tells Bill that it's his fault that he died. The others tell Bill that Georgie wouldn't blame him and that it's another of Its tricks. Bill tells his "brother" that it wasn't his fault that Georgie died, and It fades away. Bill calls on It to show itself, and Pennywise's face appears on the wall. It tells them to go while they can, and that they still don't know what It really looks like. The clown warns them that if they ever see Its true face, they'll lose their minds to the deadlights.
Richie warns that they can't fight the disembodied It, but Bill figures that it has to turn solid to feed. He puts the newspaper boat back in the channel and it floats down the stream. They follow it into a series of tunnels and a small wooden door surrounded by wooden bones. As they prepare to go in, Eddie admits that he lied and that he isn't married, and in fact has never been with a woman. He couldn't sleep with someone he didn't love, and the only people he's loved have been his six friends.
After sharing a final hug, the Losers go through the wooden door. On the other side is a huge cavern filled with web-wrapped corpses... and a comatose Audra. Bill starts to run to her, but It appears enters the cavern in Its true form: a giant spider-like creature. Bill advances on the creature while Beverly fires the first earring. She misses, and Bill recites the sentence his mother gave him to overcome his stuttering. Beverly shoots again but the earring glances off Its armored hide. The spider-creature rears up, revealing the deadlights on its underside. Bill is entranced, and Ben and Richie fall under Its spell when they run to help Bill. As Beverly looks for the earrings, Eddie realizes that he is his friends' only hope. He runs forward and sprays It with his asthma inhaler, believing that it's battery acid. Shrieking in agony, It grabs him in its pincers and crushes him.
Beverly manages to find one of the earrings, turns, and shoots It in the deadlights. It screams in pain and drops Eddie. Bill and the others run to their friend's side and look on helplessly as he dies. The wounded It flees out of the cavern, but the four Losers refuse to let it go this time. They run after it, kick it over, and pull out Its still-beating heart. As the web falls apart and the bodies drop to the ground, Bill runs back and gets Audra. They carry Audra and Eddie out and watch from the lake, but Audra doesn't wake up.
Later, Mike writes in his journal. He admits that this time he is forgetting just like the others did, and that the only way he can remember what happened is to read his previous journal entries. Mike takes the amnesia as a sign that this time their job is complete, and considers moving on now that Derry no longer needs a lighthouse keeper.
As for the others, Richie goes back to Hollywood and films a series of comedy movies with a co-star who looks and acts just like Eddie. Ben and Beverly leave together, get married a week later, and have a child ten months later. Another curse broken.
Bill stays at Mike's house, tending to Audra. He can't remember Mike and vice versa, and soon prepares to leave with his wife. However, as they go, Bill sees Silver and has an idea. He mounts up on the pike and places Audra in front of him, and then rides to beat the devil. Streaking through the streets at top speed, Audra starts to recover. She finally wakes up from her coma and wonders where they are. Bill tells her to hang on and brakes to a stop in the middle of a four-way intersection. With things back to normal for Audra, the two of them kiss.