A sinister shape-shifting entity referred to as IT lurks beneath the town of Derry, Maine, primarily in the form of a clown named Pennywise. His first victim is young Georgie Denbrough.
Georgie's brother Bill and his friends, The Losers Club, search for Georgie over the summer, believing he is only missing. The boys befriend other kids who have experienced hauntings from IT, which manifests as the thing that the children fear most.
The Losers confront Pennywise in his lair and defeat him once they stop fearing him. He retreats into the darkness when he is unable to kill them. Bill then finds Georgie's raincoat, causing him to accept that his brother is dead. His friends console him.
The Losers swear a blood oath to return to Derry as adults in the event that IT comes back to claim more victims.
The events start in 1988, in the small town of Derry, Maine. Bill Denbrough's (Jaeden Lieberher) brother, Georgie (Jackson Robert Scott), is the first victim in this incarnation of Pennywise, The Dancing Clown/IT (Bill Skarsgård).
Cut to the summer of 1989, the end of the school year. Bill is still torn up by Georgie's disappearance/death. The town of Derry continues to have more missing children. Bill rounds up his "Losers' Club" friends, Richie Tozier (Finn Wolfhard), Stanley Uris (Wyatt Oleff) and Eddie Kaspbrak (Jack Dylan Grazer) to figure out what happened to Georgie and possibly the other missing children.
While searching the sewer system, Ben Hanscom (Jeremy Ray Taylor) AKA new kid falls into the pack while running away from school bullies, Henry Bowers (Nicolas Hamilton) was cutting Ben's belly while Patrick Hockstetter (Owen Teague), Victor 'Vic' Criss (Logan Thompson) and Reginald 'Belch' Huggins (Jake Sim) held him down and watched.
The Loser Club goes into town to get first aid supplies to fix Ben. They run into Beverly Marsh (Sophia Lillis) who helps them steal the items at the pharmacy as they don't have enough money.
Patrick is next to fall victim to Pennywise in the sewer system looking for Ben. You see a missing child poster of him at a July 4th event in town.
The Loser's Club now has Ben and Beverly involved in the hunt for Georgie and It. In their travels, they see Mike Hanlon's (Chosen Jacobs) bike on the side of the road next to Henry's vehicle. They save Mike and start to have a rock fight with the bullies ending with Henry on the ground defeated.
The children realize that It feeds on fear. They all have encounters with their worst fear which the adults cannot see. The manifestations ends with It as Pennywise.
Ben has knowledge of the town past which lead the group to an abandoned house on Niebolt street which use to be the town Well House. Bill goes into the house with Richie and Eddie to face It with Beverly, Stanley, Ben and Mike outside standing watch. The confrontation ends with Beverly stabbing It and Eddie breaking his arm.
While the group is divided about going back to the house, It has taken Beverly so they gather supplies and head into the house and go down the well where Bill saw It retreat back to.
Meanwhile, Henry has killed his officer father Oscar 'Butch' Bowers (Stuart Hughes) with his own knife which he lost while chasing Ben. It had a hand in enraging Henry as we see in past scenes.
The boys start going down the well. Mike is the last to go down when Henry barges in. A scuffle ensues and Henry goes down the well along with Mike's extra animal bolt bullets.
Bill is lured away when he sees Georgie. Bill comes across It's lair with Beverly untouched but entranced and floating. There are other victims and stuff floating and in a pile. Bill continues after Georgie. The boys enter the lair and Ben rescues Beverly by kissing her. Bill confronts one-arm Georgie and shoots him in the head with the animal bolt gun which only has one bullet. This reveals that Georgie was a shell for It. The 'final' battle occurs with the children hitting, biting, tackling and gouging It. Eventually It grabs Bill and makes the kids an offer to leave without him or die. They fight and stand up to It/fear which makes It weak. Bill states that It should die of hunger as there is no more fear in them. It obliges and slinks down into a further sewer. The floating items and people start to slowly fall. Bill finds Georgie's yellow raincoat.
The kids are outside recapping the events. Beverly states that she doesn't remember much but thinks she saw a vision of them all back in the lair as adults. Bill gets the group to take a blood oath swear that if It isn't dead they will all come back to the same spot. The children start to drift off leaving Bill and Beverly. Beverly is moving to Portland to be with her aunt. Beverly and Bill have a passionate kiss.