In the small town of Derry, Maine, a group of seven children come together over a summer to confront a series of bizarre and violent occurrences. Bound by their outcast status and a shared encounter with a malevolent force, they form the Losers' Club. Each child has faced an unspeakable horror that takes the form of their deepest fears, often manifesting as a malevolent clown named Pennywise. Together, they discover that this entity has plagued the town for centuries, surfacing every 30 years to prey on the town's children. The Losers' Club vows to stop this cycle of terror, but their investigation leads them into the sewers beneath Derry where they confront the entity.

Years later, the childhood friends have grown apart, their traumatic experiences buried deep in their memories. However, when the cycle of terror returns to Derry, the now-adult members of the Losers' Club receive a devastating phone call that summons them back to their childhood town. Despite their fears and after much hesitation, they reunite to fulfill the promise they made to each other as children. As they reconvene, they must face their past traumas and piece together their fragmented memories to uncover the truth about what they encountered all those years ago, all the while knowing they must confront the monstrous entity that terrorized them in their youth once more.