In 2016, Derry, Maine, Don Hagarty witnesses his boyfriend Adrian Mellon being murdered by Pennywise after a gang of homophobic youths beat them up and throw Adrian off a bridge while leaving the annual Derry carnival. Overhearing a police scanner and then investigating the scene, Mike Hanlon discovers It has returned, and calls his childhood friends, Bill Denbrough, Ben Hanscom, Beverly Marsh, Richie Tozier, Eddie Kaspbrak, and Stanley Uris, back to Derry to honor the promise they made in 1989 to kill It if it came back. While the others travel to Derry with only hazy memories and a sense of dread, Stan commits suicide by slashing his wrists in the bathtub soon after getting the call. The Losers meet for dinner at a Chinese restaurant, where Mike refreshes their memories until they are tormented by hallucinations and learn of Stan's death.

Richie and Eddie decide to leave until Beverly reveals that ever since she was exposed to It's Deadlights, she has been experiencing visions of their deaths should they fail or leave town without fulfilling their promise. Meanwhile, Henry Bowers, who survived being pushed into the well at the Neibolt house and was arrested for murdering his father in 1989, escapes from a mental institution with help from It, in the form of the corpse of Patrick Hockstetter. At a baseball game, Pennywise kills a little girl named Victoria after deceiving her into believing he can remove a birthmark from her face. Mike...