In 2016 Derry, Maine, Don Hagarty witnesses his boyfriend Adrian Mellon being murdered by Pennywise after a gang of homophobic teenagers throw Adrian off a bridge while leaving a carnival. This convinces Mike Hanlon to call his childhood friends back to Derry and honor the promise they made in 1989 to kill It if it came back. While the others travel to Derry with only hazy memories and a sense of dread, Stanley Uris commits suicide in the bath soon after getting the call. The Losers meet for dinner at a Chinese restaurant, where Mike helps them remember It while they are tormented by hallucinations and learning of Stan’s suicide.
Richie Tozier and Eddie Kaspbrak consider leaving while Mike convinces Bill Denbrough, via a drug-induced vision, that the Shokopiwah Native American tribe’s Ritual of Chüd can stop It for good. After the others agree to perform the ritual, Beverly Marsh reveals her exposure to It’s Deadlights has her experience visions of the Losers’ deaths should they leave town without fulfilling their promise, Mike explains that the ritual requires seven personal items from their past. Meanwhile, Henry Bowers, having survived his apparent death in 1989, escapes from a mental institution with help from Pennywise. A little girl named Victoria is killed after It told her that he could remove a birthmark from her face.
Getting Stan’s shower cap in the Losers’ old clubhouse, Mike advises the others to find their artifacts by retracing their steps following their falling out after first entering Neibolt House. Beverly finds Ben Hanscom’s love letter at her old home, still believing Bill wrote it, and encounters It posing as Mrs. Kersh. Ben recalls his childhood encounter with It at high school before realizing his artifact is the yearbook page Beverly signed which he kept in his wallet. Both Richie and Eddie recall their own personal encounters with It while retrieving their artifacts: a game token from an abandoned arcade, and an inhaler. Pennywise taunts Richie before menacing Eddie in the guise of the Leper, only to flee when Eddie standing up to It causes the monster to shrink in size.
Bill finds his childhood bicycle “Silver” and recovers the paper boat from the storm drain where Georgie was killed while meeting a boy named Dean, who lives at his old house and hears voices from the shower drain. Bill regroups with the others before running off to the carnival after learning It is going after Dean, only to end up watching helplessly as It brutally devours the boy in front of him. Meanwhile, Henry unsuccessfully attempts to murder Eddie before attacking Mike, only to be killed by Richie. The Losers join a traumatized Bill at the Neibolt house, talking him out of facing It alone out of guilt.
The group descends into an underground cavern beneath the sewers, Mike providing a rock from the Losers’ fight with the Bowers Gang as they perform the ritual in the remains of the meteor that brought It to Earth ages ago. The ritual appeared to work in trapping the Deadlights in the sealing jar, but they escape after Pennywise emerges from the jar in a giant spider-like form. Pennywise pressures Mike into revealing that It killed those who previously performed the ritual, Mike assuring the others they have a better chance in facing their fears. It attacks the Losers and places Bill, Ben, and Beverly in nightmarish scenarios, which they escape once Bill lets go of his guilt over Georgie’s death and Beverly realizes Ben wrote the love letter. Richie gets caught in Its Deadlights while attempting to save Mike, and Eddie is fatally wounded after saving him. The Losers regroup and realize they can stop Pennywise by exploiting It’s subjection to having a physical form and their perception of It. Their taunting works in shrinking Pennywise into a helpless infant-sized form, killing It for good when Mike rips out Pennywise’s heart so he and the Losers can crush it. Eddie dies from his injuries, and the Losers are forced to leave him behind when It’s death causes a cave-in that destroys the Neibolt house.
The remaining Losers comfort a tearful Richie over losing Eddie, their ordeal’s end signified by the scars on their hands disappearing. The Losers part ways while retaining their memories, Ben and Beverly becoming a couple while Richie returns to the kissing bridge where he had once carved his and another’s initials, now revealed to be Eddie. Bill begins writing his new story before receiving a call from Mike as he leaves Derry, learning that Stan sent them all posthumous letters. Stan reveals in his letter that his suicide was actually a means to give his friends a fighting chance against It and asks the remaining Losers to live life to the fullest.