The film opens in October 1988 with Bill Denbrough and his seven-year-old brother, Georgie, making a sailboat out of a sheet of paper from Bill's notebook. Georgie, as his older brother is bedridden, then has to go to the basement to retrieve the paraffin, in order to make the boat float. There is a tense scene in which Georgie stares, in horror, at a pair of red eyes glaring at him. But, as he shines his torch at them, they are revealed to be a pair of two light bulbs. After Georgie and his brother coat the boat with paraffin, Georgie takes the boat out onto the flooded street, chasing it with glee until it accidentally sails down the gutter. As Georgie peers into the drain, he meets a strange being who introduces himself as "Pennywise The Dancing Clown". Pennywise claims that the storm blew him and the whole circus into the sewers, and offers Georgie a balloon and a visit to the circus. When Georgie reaches in to try to get the boat back, Pennywise grabs his arm and bites it off in one fluid motion of misshapen, nightmarish teeth; Georgie tries to run away, screaming in terror, but is dragged into the sewer, crying for Bill, to his death, traces of blood swirling in the rainwater on the street in his wake.
Eight months later, in June 1989, it is the last day of term at Derry High School. Bill and his friends Richie Tozier, Eddie Kaspbrak, and Stan Uris run afoul of bully Henry Bowers and his gang. At the same time, Beverly Marsh, a young girl who has a sexually abusive father and is bullied mercilessly by mean girls led by Gretta Keene for being a "slut", runs into a kind, overweight boy named Ben Hanscom, who is secretly in love with her, and will later write her an anonymous poem on the back of a postcard.
While making a delivery to a local butcher shop for his grandfather, home-schooled Mike Hanlon encounters Pennywise, seeing the nightmarish horror of people seemingly burn to death before his eyes, before nearly getting run over by Henry and his gang. When he looks back to the door, the illusion is gone, and the butcher's shop has returned to normal. Bill, still haunted by Georgie's disappearance and the resulting neglect from his grief-stricken parents, discovers that his brother's body may have ended up in a marshy wasteland connected by the sewers, known as the Barrens. Ben Hanscom heads to the library where he finds a book on Derry's history, learning the town has been plagued by mysterious unexplained tragedies and child disappearances for centuries. He is lured into the basement by Pennywise in the form of Easter eggs, where he is chased by a headless boy who had died from the gas explosion Easter egg hunt, whom he narrowly escapes. Pennywise also goes after Stan in the form of a living painting with misshapen eyes and a horrifying mouth, permanently traumatizing him.
Henry and his gang ambush Ben and torture him, with Henry attempting to carve his name into Ben's belly with a knife. Ben escapes and flees into the Barrens, at the same time that Bill, Richie, Eddie, and Stan discover the sneaker of a missing girl named Betty Ripsom whilst still searching for Georgie. One of Bowers' friends, Patrick Hockstetter, is slaughtered by Pennywise, who appears in the form of a horde of zombies, whilst wandering in the sewers looking for Ben. Ben, bloodied and bruised, stumbles across Bill and the other boys, who take Ben to the local pharmacy to help him, where they meet and befriend Beverly, who distracts Mr. Keene to help the boys steal medical supplies when it turns out they don't have enough money.
On his way home, Eddie is attacked by Pennywise in the form of a rotting leper while passing the abandoned house on Neibolt Street. Beverly hears the voices of several missing children, including Patrick and Betty, coming from her bathroom sink. Curious, she lowers a measuring tape into the sink, watching it descend for a long while, horrified to see it come up covered in blood and hair. Suddenly, tendrils of her cut hair shoot up from the drain, choking Beverly and hauling her closer and closer to the sink. She tries to pull away, and a fountain of blood bursts forth, soaking her and the entire bathroom in dripping crimson. Her father comes in to investigate her screaming, and sees her curled up in a corner, muttering and shaking. He can't see what's so deviously wrong, and Beverly realizes that he is blind to the blood on the walls. Bill is lured into the basement of his house at night by what he thinks is Georgie, but is really a disguised Pennywise. The group, who now refer to themselves as "The Losers' Club," each come to realize that they are being terrorized by the same creature. They then discover Henry's gang bullying Mike and chase them off with an epic rock war before befriending him.
A few weeks later, while meeting up at Bill's garage, the group deduces that Pennywise is using the sewers to move around unseen. But as the projector moves around the slides, Pennywise begins to appear in place of a family member, and begins crawling out of the screen as a gargantuan version of himself, and attempts to kill them, but fails. After narrowly escaping the attack by Pennywise in Bill's garage, the Losers Club go to the house on Neibolt Street where Bill deduces "It" to be hiding. The creature uses its shape-shifting abilities to separate the group and attempts to pick them off. Richie is terrorized by a room of horrible clown-dolls, one of which is of himself, in a coffin. He barely escapes in time, and he and Eddie come across the illusion of the still-living, half-devoured corpse of Betty Ripsom. Eddie breaks his arm by falling through a hole upstairs. It emerges and prepares to eat Eddie, but Bill and Richie intervene. After taunting Bill about Georgie, Pennywise tries to kill him and Richie, only to be brutally impaled through the head by Beverly with a fence post. The clown slashes Ben before fleeing down the well of the Mansion. After the encounter, however, the group begins to splinter, And Eddie, whose arm badly broken, is taken to the hospital by his mother and is said not to hang out with them anymore, with only Bill and Beverly still resolute in fighting It. Eventually, he fights against this, attempting to repeat what an intern at the chemist told him about his medication when his mother tells him he's getting over an illness; "They're GAZEBOS! They're bullshit!" (He meant to say 'Placebo').
One night in August, Beverly's father finds Ben's anonymous, romantic poem to her, and attempts to rape her, but is incapacitated when Beverly hits him in the side of the head with a toilet cover. Presumably, he is knocked out; hopefully, he is dead. She is then ambushed and kidnapped by Pennywise. Bill uses the opportunity to reassemble the Losers and mount a rescue. "It" responds back by giving Henry, who is gradually losing his sanity, a switchblade while compelling him to murder his abusive father, before sending him to intercept the Losers. Beverly awakens in the sewers and attempts to escape, but is caught by Pennywise, who renders her Catatonic with his Deadlights. At Neibolt House, Henry arrives and tries to kill Mike first, but Mike manages to throw him down the well, where he plummets to his supposed death. Pennywise traps Stan in his worst fear and tries to eat him, but fails when the other Losers intervene with his plan. They then find Pennywise's Lair, where they find Beverly, in her catatonic state and floating in suspended animation, with the bodies of the other missing children kept in a similar form of suspended animation. Ben kisses Beverly, which restores her to normal.
Pennywise tries to trick Bill by posing as Georgie, but the deception fails when Bill shoots "Georgie" in the head with the bolt gun and tells him "You're not Georgie". After a brief battle with the Losers, Pennywise takes Bill hostage and offers to spare the others if they let him go and eat Bill in peace. Richie begins stating all the reasons they should leave Bill and save themselves, but then grabs a bat and says "And now I've gotta kill this fucking clown." The Losers no longer fear "It" anymore, and begin fighting as one fearsome unit. After a brutal battle, they all defeat Pennywise, who descends back into the well and goes into hibernation again. The suspended children, the ones who've been missing for years as Pennywise's victims, float back down to the ground as the Losers Club Members discover Georgie's yellow raincoat jacket. Bill's friends comfort him as he cries, emotionally breaking down from accepting Georgie's death. One month later, Beverly informs the group of a vision she had of them fighting Pennywise 27 years later, and the Losers form a blood oath in a circle, promising that they will each return to Derry should Pennywise resurface to kill him for good. After departing one by one, Bill kisses Beverly. As the end credits roll, a title card appears reading "It: Chapter One", this dictates that the story of "It" will continue in a hopefully amazing sequel, "It: Chapter Two with a little scary It laugh"
- Bill Skarsgard as Pennywise / IT
- Jaeden Lieberher as Bill Denbrough
- Finn Wolfhard as Richie Tozier
- Sophia Lillis as Beverly Marsh
- Jeremy Ray Taylor as Ben Hanscom
- Chosen Jacobs as Mike Hanlon
- Jack Dylan Grazer as Eddie Kaspbrak
- Wyatt Oleff as Stan Uris
- Javier Botet as The Leper
- Nicholas Hamilton as Henry Bowers
- Owen Teague as Patrick Hockstetter
- Logan Thompson as Victor "Vic" Criss
- Jake Sim as Reginald "Belch" Huggins
- Megan Charpentier as Gretta Bowie
- Jackson Robert Scott as Georgie Denbrough
- Pip Dwyer as Sharon Denbrough
- Geoffrey Pounsett as Zack Denbrough
- Steven Williams as Leroy Hanlon
- Stuart Hughes as Oscar Bowers
- Stephen Bogaert as Al Marsh
- Ari Cohen as Rabbi Uris
- David Katzenberg as El Aparato
- Tatum Lee as Judith
- Anthony Ulc as The Butcher
- Carter Musselman as Headless Boy
- Katie Lunman as Betty Ripsom
- Cyndy Day as Pharmacy Cashier
- Edie Inksetter as The Hostess
Changes from the novel and the miniseries
- Unlike the novel (which takes place in 1957 and 1958) and the miniseries (which takes place in 1960), the movie takes place in 1988 and 1989.
- Mike's parents are mentioned to have burned to death in the fire at the Black Spot, while in the novel they are still alive.
- Rather than being aged 11-12 and attending elementary-middle school, like in the novel and original adaptation, the adolescents of the Losers' Club and Bowers Gang are teenagers attending Derry High School, the Losers being aged 12-13 and in the 7th grade, whilst Henry's gang is 15-16 and in the 10th grade.
- Patrick Hockstetter is portrayed somewhat differently from the novel: in addition to there being no mention or showing of him having a deceased younger brother, refrigerator full of dead animals, or pencil box full of dead flies, he is taller and ganglier than any of his friends (including Belch, who was originally the biggest of them by stature) and wields a makeshift blowtorch using a lighter and a can of hairspray. As well as this, rather than having a sole fear of flying leeches, Patrick is shown to be afraid of zombies.
- Rather than being six feet tall as in the novel and miniseries, Belch is shorter and more stockily-built, being dwarfed in terms of height by the gangly Patrick Hockstetter (also contrary to the novel).
- In the book, Patrick is kidnapped by It in the form of flying leeches and officially dies when he wakes up to find It beginning to feed on him. In the film, It terrorizes Patrick in the form of zombies of the missing kids of Derry while he was searching the sewer for Ben, chases him into a dead end, and slaughters him in the form of Pennywise.
- The cattle gun was a replacement for the slingshot in the novel.
- Georgie is dragged into the sewers by Pennywise, and devoured, after getting his arm bitten off, instead of being left to bleed to death like in the novel and miniseries. Additionally, the sole witness of Georgie's fate, rather than a neighborhood man named Dave Gardener who heard his screams of agony, is an unnamed possibly elderly woman and her pet cat who witnessed the whole thing from the porch of a nearby house.
- This adaptation features characters who didn't appear in the miniseries such as Patrick Hockstetter, Greta Bowie (here by the last name of 'Keene'), Butch Bowers, and Eddie Corcoran. Exceptions include Gard Jagermeyer, Marcia Fadden, Mike's parents, and the characters who only made it as far as the miniseries such as Peter Gordon, Moose Sadler, Arlene Hanscom, Ms. Douglas, and Captain Hanscom.
- Victor and Belch aren't killed by Pennywise since they weren't seen with Henry when he tries to kill the Losers, indicating that they survived the entire film, unlike in the novel and miniseries.
- However, In a deleted scene from the 2017 film, Henry killed both Belch and Vic by slitting their necks with his knife.
- Like the miniseries, the scene where Bev has sex with all of the boys is omitted and is changed to a blood pact.
- Pennywise takes Bill hostage in the movie, whilst in the miniseries, IT takes Stan hostage.
- Like in the miniseries, Maturin the Turtle doesn't appear, but instead, he is referenced, such as a short the Lego turtle Bill drops, and in the quarry scene where Bill tells the others that there's a turtle in the water.
- Henry's father is an alcoholic cop instead of an ex-marine. His abusive behavior has been reduced drastically, and most of his traits from the novel have been omitted in the film.
- Like the miniseries, the handjob scene between Henry and Patrick has been omitted.
- The number 27 is involved a lot in this film. This is the second adaption of novel IT, being released 27 years after the 1990 Miniseries. The actor who played Pennywise (Bill Skarsgård) turned 27 just a little more than a month before the movie's release date. This pays homage to the novel, in which it is stated that "It" returns every 27 years.
- IT is currently the highest grossing Stephen King horror film as of September 2017, following the reason the film grossed so far over $630 million worldwide.
- As Georgie is about to die, there is a cat that witnesses it. Likely a reference to "Cat's Eye", a movie based off two of King's short stories: Quitters, Inc. and The Ledge.
- The film makes a reference to thrash metal bands, Metallica and Anthrax; Reginald "Belch" Huggins is shown wearing a shirt for the two.
- A deleted scene shows that Georgie escapes his death by grabbing the paper boat too fast for Pennywise to catch his arm and leaving.
- Another deleted scene shows an alternate ending when Bill arrives back home. He shows his mother the scar on his hand and as they drive off, the camera zooms to a drain as we hear some raindrops.
Filming for It: Chapter Two is due in 2018. This film, which will focus on the Losers returning to Derry to fight Pennywise as adults, has been confirmed and will be released on September 6, 2019. The actors who played The Losers Club in Chapter One will make an appearance as flashbacks.