In October 1988, stuttering teenager Bill Denbrough gives his seven-year-old brother, Georgie, a paper sailboat. Georgie sails the boat along the rainy streets of small town Derry and is disappointed when it falls down a storm drain. As he attempts to retrieve it, Georgie sees a clown in the sewer who introduces himself as "Pennywise the Dancing Clown". The clown entices Georgie to come closer by talking about the circus, then severs his arm and drags him into the sewer drain.
The following summer, Bill and his friends (loudmouth Richie Tozier, hypochondriac Eddie Kaspbrak, and timid Stan Uris) run afoul of bully Henry Bowers and his gang. Bill, still haunted by Georgie's disappearance and the resulting neglect from his grief-stricken parents, discovers that his brother's body may have washed up in a marshy wasteland called the Barrens. He recruits his friends to check it out, believing his brother may still be alive.
"New kid" Ben Hanscom learns that the town has been plagued by unexplained tragedies and child disappearances for centuries, and is lured into the basement of the library by Pennywise, before escaping the library. He is attacked by Bowers' gang for being fat and has an 'H' carved into his stomach. He is able to escape before Henry can finish his name and flees into the Barrens and meets Bill's group. They find the sneaker of a missing girl named Betty Ripsom, while a member of the pursuing Bowers Gang, Patrick Hockstetter, is killed by Pennywise looking for Ben. The Losers see Ben bloody, and rush to the nearest drug store to fix him up. They don't have enough money, however, and Beverly Marsh, a classmate and friend of Ben bullied by mean girls over rumors of promiscuity, helps them in stealing the products.
Beverly helps clean Ben's wound and joins the group; both Bill and Ben develop feelings for her. Later, the group befriends homeschooled black kid Mike Hanlon after defending him from Bowers in a rock war. All the while each member of the group has encountered terrifying phenomena in various forms; these include a menacing clown for Richie, a headless boy for Ben, a fountain of blood for Beverly, a diseased and rotting man known as a Leper for Eddie, a creepy painting come to life for Stan, Mike's parents burning alive, and finally a phantom Georgie for Bill.
Now calling themselves "The Losers Club", they realize they are all being terrorized by the same entity. They determine that Pennywise (or "It") assumes the appearance of what they fear, awakens every 27 years to feed on the children of Derry before returning to hibernation, and moves about by using sewer lines – which all lead to the town's original well under the creepy, abandoned house at 29 Neibolt Street. After an attack by Pennywise, the group ventures to the house to confront him, only to be separated and terrorized. Bill and Richie leave Eddie behind, and Eddie sees the leper. Richie then gets locked in a room full of clown dolls, and sees a casket with a doll of himself inside. Pennywise jumps out from behind the casket and chases Richie out the door. They realize Eddie is gone, and find him right before Pennywise kills him. Eddie breaks his arm, while Pennywise gloats to Bill about Georgie. As they regroup, Beverly impales Pennywise through the head, forcing the clown to retreat. However, after the encounter Eddie is forbidden to see the group by his overprotective mother, Richie argues that fighting It isn't worth their lives, leading him to say Georgie is indeed dead in which Bill punches Richie in the face. Richie, Stan and Mike leave the group out of fear, and only Bill, Ben, and Beverly remain resolute in fighting It and prepare for another confrontation.
Weeks later, after Beverly confronts and incapacitates her abusive father, she is abducted by Pennywise. The Losers' Club reassembles and travels back to the Neibolt house to rescue her. Henry Bowers, who has killed his father after being compelled into madness by It, attacks the group. Mike fights back and pushes Henry down the well to his apparent death. Amidst, Beverly wakes up in Pennywise's lair. She tries to run away, but Pennywise catches her, and shows her It's true form; the Deadlights. The Losers descend into the sewers and Stan is separated and almost killed by It in the form of the "Flute Lady", but is rescued by the Losers. They find It's underground lair, which contains a mountain of decayed circus props and children's belongings, around which the bodies of missing children float in mid-air. Beverly, now catatonic after being exposed to It's true form, is restored to consciousness as Ben kisses her. Bill encounters Georgie, but recognizes that he is Pennywise in disguise, and shoots him with Mike's bolt gun. Pennywise attacks the group and takes Bill hostage, offering to spare the others if they let It keep Bill. Richie lists bad things Bill has done to him, but realizes friendship is important, and attacks Pennywise with a baseball bat. The Losers see their various fears, but wound It quickly. After a brutal battle they defeat Pennywise and he retreats, with Bill declaring that It will starve during its hibernation. Their victory is bittersweet, as Bill finds his brother's raincoat and accepts his death while his friends comfort him and the missing children's bodies float to the ground. The Losers then leave the sewers.
As summer ends, Beverly informs the group of a vision she had while catatonic, where she saw them fighting It as adults. The Losers swear a blood oath that they will return to Derry as adults if It returns and destroy the creature once and for all by cutting their palms with a glass shard. Stanley, Eddie, Richie, Mike, and Ben make their goodbyes as the group disperses. Beverly tells Bill she is leaving the next day to live with her aunt in Portland. Before she leaves, Bill reveals his feelings and they share a kiss.
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