In October 1988, Bill Denbrough crafts a paper sailboat for his younger brother, Georgie, who is just seven years old. Georgie enjoys sailing the boat through the rainy streets, but it unfortunately becomes lost in a storm drain. As he tries to retrieve it, he encounters a sinister clown named Pennywise the Dancing Clown, who cruelly bites off Georgie's arm and drags him into the depths of the sewer.
The next summer, Bill, along with his friends Richie Tozier, Eddie Kaspbrak, and Stanley Uris, find themselves facing off against older bullies, including Henry Bowers, Belch Huggins, Patrick Hockstetter, and Victor Criss. Bill suspects that his brother's body may have ended up in the marshy area known as "The Barrens". Driven by hope, he and his friends embark on a search, convinced that Georgie might still be alive. Meanwhile, Bill's schoolmate Ben Hanscom uncovers a chilling history of unexplained tragedies and disappearances that have haunted their town for centuries. During their search, Ben encounters Bill's group in the Barrens. They soon discover a sneaker belonging to missing girl Betty Ripsom, and Patrick meets a grim fate at the hands of drowned children while searching the sewers.
As the summer unfolds, Bill and Ben find themselves falling for Beverly Marsh, a girl who is frequently bullied. They also befriend orphan Mike Hanlon, whom they rescue from Bowers' gang. Each member of what they call "The Losers Club" has their own horrifying encounters with Pennywise: Ben is pursued by a headless undead boy, Beverly faces a blood-spewing sink visible only to children, Eddie is chased by a decaying leper, Stan witnesses a painting of a woman coming to life, Mike relives the traumatic memory of his parents in a fiery incident, and Bill is haunted by a phantom of Georgie. They soon realize that they are all being pursued by a single entity, which they name "It". This malevolent force manifests as their individual worst fears and awakens every 27 years to feast on the children of Derry, traveling through the town's sewer lines that converge at an old well beneath an abandoned house on Neibolt Street.
During an encounter in Bill's garage, Pennywise attacks as the group examines photographs. Out of all of them, Beverly remains remarkably fearless in the face of the clown. They resolve to confront him, though they soon find themselves separated and besieged by their deepest fears. As Pennywise mocks Bill about Georgie, he charges at them, but Beverly bravely stabs him in the head with a metal spike. Despite his injuries, Pennywise retreats after attacking Ben, leaving only Bill and Beverly to fend against the clown.
Weeks later, after Beverly confronts her abusive father, Pennywise abducts her. In a bid to rescue her, The Losers Club returns to the abandoned house. Following the brutal fallout from Pennywise’s influence on Bowers, who has gone insane and murdered his father, a confrontation ensues; Mike manages to push Bowers down the well. The group then descends into the sewers, where they discover Pennywise's lair, littered with decaying circus props and children's belongings, floating around the bodies of his child victims. After Beverly falls into a catatonic state due to exposure to the Deadlights within Pennywise’s mouth, she regains consciousness when Ben kisses her. Bill encounters Georgie but realizes that he is actually Pennywise in disguise. Pennywise then holds Bill hostage, trying to negotiate a promise where he would spare the others if they leave him alone with Bill. The Losers reject the offer, launching into a fierce battle against Pennywise while overcoming their individual fears as he cycles through various manifestations. Ultimately, they overpower Pennywise when Beverly stabs him in the face with a broken pipe, forcing him to retreat down the well. Bill finds a bloodstained raincoat belonging to Georgie, and the group comes together to comfort him as he breaks down in tears.
As summer comes to an end, Beverly shares a vision that hints at their future confrontation with Pennywise as adults. The Losers affirm a blood oath to return if Pennywise ever resurfaces. After saying their goodbyes and parting ways, Beverly and Bill discuss her upcoming move to live with her aunt, culminating in a heartfelt moment where Bill confesses his feelings, leading to a kiss between them.
