It

In October 1988, twelve-year-old Bill Denbrough makes a paper sailboat for his seven-year-old brother Georgie. Georgie sails the boat along rainy streets, only to have it fall down a storm drain. Attempting to retrieve it, Georgie sees a clown in the drain, who introduces himself as "Pennywise the Dancing Clown". Pennywise bites off Georgie's arm and drags him into the sewer.

The following summer, Bill and his friends Richie Tozier, Eddie Kaspbrak, and Stanley Uris run afoul of older bullies Henry Bowers, Belch Huggins, Patrick Hockstetter, and Victor Criss. Bill calculates that his brother's body may have washed up in the marshy wasteland "The Barrens". He investigates with his friends, believing Georgie may still be alive. Bill's classmate Ben Hanscom learns that unexplained tragedies and child disappearances have plagued the town for centuries. Fleeing Bowers' posse, Ben meets Bill's group in the Barrens. They find the sneaker of missing girl Betty Ripsom, and Patrick is killed by drowned children while searching the sewers.

Bill and Ben develop feelings for bullied girl Beverly Marsh. The group befriends orphan Mike Hanlon having rescued him from Bowers. Each member of the group has encountered terrifying manifestations of Pennywise: Ben is chased by a headless undead boy, Beverly is attacked by a sink spewing blood only children can see, Eddie is chased by a rotting leper, Stan witnesses a painting of a woman coming alive, Mike relives his parents burning alive, and a phantom of Georgie haunts Bill. Dubbing themselves "The Losers Club", they realize they are all being stalked by the same entity, which they call as "It". They determine that It appears as their individual worst fears, awakening from hibernation every 27 years to feed on the children of Derry, and that it travels via the sewer lines, which all lead to an old well under an abandoned house on Neibolt Street. Pennywise attacks them while they look through photos in Bill's garage. Beverly shows little fear of Pennywise. The group ventures to confront him, only to be separated and terrorized by their fears. As Pennywise gloats to Bill about Georgie, It charges at them, but Beverly stabs him in the head with a metal spike. Pennywise retreats after injuring Ben. The group flees with only Bill and Beverly fighting Pennywise.

Weeks later, after Beverly confronts and incapacitates her sexually abusive father, Pennywise abducts her. The Losers Club returns to the abandoned house to rescue her. Bowers, who has murdered his abusive father after being driven insane by Pennywise, attacks the group; Mike pushes Bowers down the well. The Losers descend into the sewers and find Pennywise's lair, which contains decayed circus props and children's belongings, around which the bodies of It's child victims float in mid-air. Beverly, catatonic after being exposed to Deadlights inside It's gaping mouth, is restored to consciousness when Ben kisses her. Bill encounters Georgie, but realizes he is Pennywise in disguise. Pennywise takes Bill hostage, offering to spare the others and go into hibernation if they leave him Bill. The Losers reject this, battling Pennywise while overcoming their various fears as the creature cycles through them. The Losers beat Pennywise before Beverly stabs him in the face with a broken pipe. It retreats, falling backwards down a well. Bill finds Georgie's bloodstained raincoat, and the group comfort him as he cries.

As summer ends, Beverly informs the group of a vision she had while catatonic, of them fighting Pennywise again as adults. The Losers swear a blood oath that they will return if Pennywise ever returns. After the others make their goodbyes and disperse, Beverly and Bill discuss her moving away to live with her aunt. Bill reveals his feelings and they kiss.