Derry, Maine, October 1988
Bill Denbrough (Jaeden Lieberher) helps his little brother Georgie (Jackson Robert Scott) make a paper sailboat, calling it the S.S. Georgie. Georgie wants to go outside to sail it in the pouring rain, but Bill is too sick to join him. The brothers hug before Georgie runs out to play.
Georgie sails his boat down the street. He chases after it and accidentally runs into a roadblock. The boat sails to the end of the street until it falls into the sewer. Georgie runs to try and get it, worried that Bill will be mad. As he looks into the sewer, a pair of yellow eyes emerge. They belong to an entity calling itself Pennywise the dancing clown (Bill Skarsgard). Pennywise playfully speaks to Georgie before offering him his boat back. Georgie reaches for it, only for Pennywise to grab his arm and sink his massive teeth into it. The creature rips off Georgie’s arm and leaves him crying for Bill before dragging him down into the sewers.
Eight months later. June 1989
We meet Mike Hanlon (Chosen Jacobs) as his grandfather Leroy (Steven Williams) is making him kill a sheep with a nail gun. Leroy lectures the boy before doing the deed himself.
It’s the last day of school. Bill and his friend Richie Tozier (Finn Wolfhard), Eddie Kaspbrak (Jack Dylan Grazer), and Stanley Uris (Wyatt Oleff), who all form The Losers Club, are heading out to start their summer. Unfortunately, they run into the school’s psychopathic bully Henry Bowers (Nicholas Hamilton) and his goons Patrick Hockstetter (Owen Teague), Victor Criss (Logan Thompson), and Belch Huggins (Jake Sim). Bill tells Henry he sucks after the punk mocks his friends, and Henry approaches Bill threateningly until he sees his father, Officer Bowers (Stuart Hughes), standing behind Bill. He walks away, but not before licking his palm and smearing it on Bill’s face.
A girl named Beverly Marsh (Sophia Lillis) is smoking in the bathroom. A group of mean girls led by Gretta (Megan Charpentier) taunt her and accuse Bev of being a slut. One girl fills a trash bag full of water and dumps it on Bev, but she covers her head with her book. On her way out of the building, Beverly meets the new kid, Ben Hascom (Jeremy Ray Taylor). He gets picked on for his weight and he doesn’t have any friends. Bev signs his yearbook, and Ben appears to develop a crush on her.
When Bill gets home, his father Zach (Geoffrey Pounsett) scolds him for creating a model with tubes of the sewer system in Derry since he still believes that Georgie is only missing. Zach tells him to accept that Georgie is dead.
Mike goes to the butcher shop to make a delivery. Behind the door around the alley, he hears what sounds like screaming and pounding. The door opens and several burnt hands start sticking out. Mike then sees Pennywise, but is nearly run over by the Bowers gang in a car. Mike flees.
Stan is at the synagogue studying for his bar mitzvah. There is a painting in the rabbi’s office that creeps him out showing a woman with a distorted face. The painting falls, and when Stan picks it up, the woman is gone. He turns around and IT manifests as the woman with a fanged smile. Stan runs away.
Ben is at the library looking up Derry’s history. From a series of articles, he learns that children in Derry have been disappearing under mysterious circumstances for centuries. The book contains a depiction of Pennywise and a missing boy’s head in a tree. Ben is then lured into the basement where he sees the headless boy before getting chased out by Pennywise.
On his way home, Ben is attacked by the Bowers gang. Henry’s goons hold him against the bridge railing. Patrick wants to burn Ben with a lighter and bug spray, but Henry plans to carve his name into Ben’s stomach. An old couple drive by and see this but do nothing (it’s implied that IT is there with the presence of a red balloon). Henry only cuts an H before Ben kicks him in the nuts and rolls over the bridge and into the woods. The punks chase after Ben. Meanwhile, the Losers are near the sewers as Bill has convinced his friends to help him find Georgie. Ben falls into the water near them, and the boys grab him and bring him to safety.
Patrick runs into the sewers to try and get Ben, splitting from his friends. He encounters demonic-looking children that make him run, but he comes across a dead end. A bunch of red balloons appear before him, and they all pop to reveal Pennywise, who promptly devours Patrick.
The boys go to the pharmacy to get cotton balls and bandages, but they are short on money. Beverly is there buying tampons when she runs into the boys. After hearing their problem, Bev pretends to flirt with the pharmacist, Mr. Keene (Joe Bostick), to distract him. He comes off as a creep to her, but she manages to distract him long enough for the boys to steal their supplies, and for her to sneak away some cigarettes. Bev goes outside and joins the boys when she sees the boys tending to Ben.
Beverly goes home to her father (Stephen Bogaert), who is an even bigger creep than Mr. Keene. He comes onto his daughter, causing her to run into the bathroom crying. She then proceeds to cut off her hair until it’s at a shorter length.
The boys (now joined by Ben) are at the quarry ready to jump into the lake, but no one is eager to go first. Bev shows up and jumps first, leading the others to join. They have fun while swimming and hanging out. Later, Ben tells everyone what he read about in Derry’s history.
Eddie is walking home when he passes the abandoned house on Neibolt Street. He is suddenly attacked by a leper (Javier Botet), causing Eddie to run through the house’s yard. As he reaches the fence, he sees Pennywise trying to lure him toward him. Eddie runs under the fence and escapes.
Bev is in her room with a postcard with a poem written on it by a secret admirer. She thinks it’s Bill but it was actually Ben that wrote it. From her bathroom, Bev hears the voices of children. She looks into the sink where the voices are, and clumps of hair start reaching out to grab her before a fountain of blood gushes all over the bathroom. Mr. Marsh comes in to see Bev horrified, but he cannot see the blood. He simply comments that her hair makes her look like a boy.
Bill is walking around the house when he sees what looks like Georgie running around. Shocked, he follows Georgie to the basement, which is flooding. Georgie steps out and invites Bill to join him, saying “We all float down here. You’ll float too.” He repeats “You’ll float too” until he yells and decomposes. Pennywise then emerges from the water and tries to get Bill, but he runs out of the basement.
The next day, the kids are riding their bikes when they see Mike’s bike and the Bowers gang’s car. They go down by the creek to find the punks harassing Mike for being black. Bev throws a rock at Henry’s head, which initiates a rock-throwing war between both groups. Mike runs over to the Losers side while Vic and Belch run away as Henry is knocked unconscious.
As the kids walk away with Mike, Bill mentions what he saw in his house. Eddie backs him up on having seen Pennywise. Mike mentions what he saw and talks about how his old house was burnt down by racist goons, and his parents died trying to break the door down to his room, with their skin having melted to the bone. Richie is the only one that hasn’t been haunted, but he admits to being terrified of clowns.
The kids go to Bill’s garage where they look over a map of Derry through a projector. They see that the sewers are linked to the Neibolt house, where IT lives. The projector then starts working itself, showing pictures of Bill and Georgie with their parents, but with Pennywise’s face appearing over their mother. They knock the projector over, but it keeps playing until Pennywise fully emerges from the screen and tries to get the kids. They manage to get out of the garage before he can harm them.
The Losers go to the Neibolt house to face IT head-on. Bill, Richie, and Eddie enter the house while the others stay outside as lookouts. Richie sees a missing poster of himself and he freaks out, but Bill reminds him it’s not real. Eddie gets separated from his friends when Pennywise finds him. He falls through the floor and lands in the kitchen, breaking his arm. Bill and Richie try to reach him through three doors. The first door they open reveals a headless girl. The second door traps Richie inside with a room full of clown dolls. He sees a coffin with a dummy of himself in it. He shuts it, and Pennywise jumps out. He tries to attack Richie, but he runs out of the room. Pennywise returns his attention to Eddie, but Bill and Richie get to him before the clown eats him. Beverly then shows up to drive a spear through Pennywise’s head, forcing him to retreat.
Eddie’s mom forbids the Losers from hanging out with him after she sees his broken arm. Bill and Richie then get into a fight when Richie says this whole pursuit of Georgie has nearly gotten them killed. The Losers part ways as the other boys are too terrified to keep going.
It’s now August, and the kids have continued going about their lives. Eddie goes to the pharmacy to pick up his asthma medication, only for Gretta (who works behind the counter) to tell him that they are placebos. She then writes “LOSER” on his arm cast.
Henry is with Vic and Belch shooting things with his dad’s gun. He orders Belch to bring him a cat to shoot, but Officer Bowers shows up to take the gun from Henry. He then shoots the ground around Henry’s feet to humiliate him in front of his friends and expose him as a coward. Later, Henry sees a red balloon on his mailbox. Inside is a knife, sent by IT. He goes inside as his father is asleep with the TV on. On the screen appears a woman with children, all urging Henry to kill his father. He does so by sticking the knife in his neck and letting him bleed out. The kids on TV (all of IT’s victims) then start chanting “KILL THEM ALL! KILL THEM ALL!”
At the Marsh house, Beverly’s dad attempts to act upon his lust toward his daughter. She fights him off and runs into the bathroom. When Mr. Marsh gets there, Bev whacks him across the face with a toilet lid, killing him. Just as she is about to leave, Pennywise finds her and takes her.
Bill goes by Bev’s house and finds her father dead in the bathroom, and sees “YOU’LL DIE IF YOU TRY” written on the wall in blood. He realizes Bev has been taken by IT, so he goes to make amends with his friends to rescue her. When they get Eddie, his mom forbids him from leaving and joining his friends, but he defies her when he confronts her over the placebos. He ditches her and joins his friends.
The boys go to the Neibolt house and find a well where IT dwells. They climb down a rope, but before Mike can head down, Henry shows up and attacks him. He starts pulling the rope up to prevent the boys from climbing back up. Henry tries to kill Mike with the nail gun that he brought, but Mike fights him off and manages to push him down the well, sending him to his apparent death.
Going further into the well, Stan is attacked by IT as the woman from the painting. She munches on his head, but the boys scare IT away and comfort a mortified Stan.
All the boys head further into the well where they find IT’s lair. All the children he’s taken are floating up in the dead-lights under some kind of trance. The boys find Bev floating. They pull her down, and Ben kisses her to break her out of the trance. Bill then sees Georgie emerge from the shadows. Georgie tells Bill he missed him and was waiting for him to come for him. Bill hugs his brother, but he knows it’s not really Georgie, and he shoots him in the head. His body writhes on the ground until he turns into Pennywise. The clown tries to kill the kids, but they fight him off until he has his hold on Bill. Pennywise gives the others the option to die together, or leave him with just Bill. Bill tells them to run, but Richie is the first to fight back. The kids then start beating the crap out of Pennywise until he is powerless, unable to hurt them because they no longer fear him. Before Bill can strike the fatal blow to the clown’s cracking skull, Pennywise retreats into the darkness and thus escapes. The kidnapped children then float downward. Bill then finds Georgie’s raincoat. He realizes that Georgie truly is gone. Bill breaks down in tears as his friends gather around and hug him.
It is now September. The Losers swear a blood oath to return to Derry in 27 years should IT ever return. They hang out for a while until each of them gradually leaves, with only Bill and Bev stay behind. She is going to move to Portland now. As she walks away, Bill runs up to her and kisses her.