Six of the seven reunite in Derry in 1985 on the strength of an old promise.
Bill, Ben, Bev, Richie, Eddie, Mike and Stan, a group of kids, form the "Losers Club," a group of social misfits who meet and form a tight-knit friendship. Each of the children individually encounter the mysterious, child-killing clown haunting their home town of Derry, Maine. The monster, which the group later collectively names It, usually appears as the thing the child victim most fears before taking the form of Pennywise the Dancing Clown. Their separate encounters are later fortified when they all witness Pennywise reach out from a photograph (threatening to kill them) in an album owned by Mike.
Spurred by Bill's desire for revenge on It for killing his younger brother Georgie, the Losers resolve to locate It's home in the sewers and destroy the threat to Derry once and for all. Henry Bowers, the psychopathic neighborhood bully, and two of his friends, Victor Criss and Belch Huggins, follow the group into the Barrens and into the sewers, in a bid to ambush and kill them as revenge for an earlier rock fight. During their trek through the sewers, Stan is pulled away from the group by Henry and Belch and Henry pulls out a switchblade. Victor has been ordered to ambush the other losers from another side but is killed by It.
As the remaining losers come to the middle of the sewers, they discover Stan is missing. Belch restrains Stan as Henry prepares to kill him with the pocketknife, but Pennywise (in the form of the "deadlights") bursts through a sewage pipe and kills Belch while Henry and Stan watch in horror. As It makes its way out of the pipe, Stan flees and Henry's hair instantly turns white from the sight of It's true form. It spares Henry's life and continues searching for Stan.
Stan meets up with the losers and warns them that It's true form has something to do with living white lights that instantly kill anyone who looks directly at them and which is far more reprehensible than Pennywise. They are to avoid looking into It's "deadlights." It vanishes, and smoke fills the chamber. The seven form a circle, although It attempts to distract and break them apart by simultaneously appearing as Georgie, Beverly's father and a werewolf. It, in the form of Pennywise, attempts to eat Stan alive, but Eddie stuns It with his asthma inhaler. Beverly cracks the monster's head open with a silver slug fired from a slingshot, revealing the bright deadlights underneath. However, before the children can kill It, It somersaults through the air and escapes through a drain in the floor. The group grabs It's arms, only for the glove to rip off, revealing a larger 3-fingered claw that disappears. After arguing and deciding It is dead, the group exits the sewers and make a promise to return if It evers resurfaces.
Mike Hanlon, the only member of the Seven who never left Derry, summons the rest of the group back to their home town. Stan commits suicide rather than face the fear of facing It again. However, this is left ambigious as the tiles in the bathtub in which he is found are smeared with blood spelling "IT", which may have also suggested It murdered Stan.
As the remaining six congregate in Derry, each has another frightening encounter with It. Henry Bowers, who had been placed in a mental hospital after accepting blame for the child deaths in the early 1960s, escapes under the influence of It (which took the form of the deceased Belch) to take revenge on the remainders of the Seven. After escaping, Henry attacks Mike at the hotel where the Seven are staying and stabs him, after which Henry accidentally and fatally stabs himself during a grapple with Eddie and Ben that saves severely wounded Mike's life. With Henry dead, It is left to complete the task of killing the Losers on its own.
At the hospital, Mike gives Bill the two silver slugs they made to use against It when they were children. Bill gathers the others and declares the slugs are "representatives" of Mike and Stan, thus the "Seven" are now reunited and ready to confront it. Bill's wife Audra follows him to Derry and is captured by It who hypnotizes her by using the deadlights. Bill, Beverly, Richie, Eddie, and Ben return to the sewers and rescue Audra, who has become catatonic. They make their way to It's lair where they discover It's true monstrous, spider-like form.
During the climax, Bill, Richie, and Ben are paralyzed by the deadlights located on It's abdomen. Eddie is grabbed and mortally wounded by It before Beverly shoots out the deadlights with her slingshot and one of the silver earrings. The others tend to Eddie, but he dies in the middle of his last sentence, then the others kill It by disemboweling it and ripping out its heart.
Richie returns to his showbiz career which extends into movies, and partners with a man who acts and behaves much like Eddie. Ben and Beverly fall in love, marry and are expecting a child. Mike is released from the hospital and remains in Derry, but considers whether he ought to move now that Derry no longer needs a "lighthouse keeper" to watch over the town for the monster's return. Bill helps Audra to come out of her catatonia by taking her on a seemingly suicidal bicycle ride impersonating the Lone Ranger, which is something he had done years earlier to help revive a young Stan, who was frozen with fear. As the film closes, Pennywise's evil laugh is heard one last time.
- Bill Denbrough
- Ben Hanscom
- Stan Uris (deceased)
- Mike Hanlon
- Eddie Kaspbrak (deceased)
- Richie Tozier
- Beverly Marsh
- 11/22/63 (only Richie and Beverly actually appear while Bill, Ben, Stan, Mike, and Eddie are mentioned briefly)
- Dreamcatcher (only the names of the Losers are shown on a sign at the site of the Standpipe)
- Insomnia (novel)(only Mike Hanlon appears).