It

In 1985, the six surviving Losers—Bill, Ben, Beverly, Eddie, Richie, and Mike—descend into the sewers beneath Derry to finish what they started as children. Along the way, external threats resolve themselves: Henry Bowers, freed from the asylum by It and sent to kill the Losers, is instead killed while attacking Eddie. Beverly's abusive husband Tom Rogan is lured by It to its lair along with Bill's wife Audra; Audra is left catatonic by the sight of It's true form, and Tom collapses and dies of shock. Mike, badly injured by Henry earlier, is left near death in a hospital, and the other five Losers pause in the sewers to psychically send him strength so he can fend off a nurse under It's control.

The group reaches It's lair and confronts the creature in its true monstrous form, a giant spider-like being. Bill and Richie attempt the Ritual of Chüd, entering It's mind, but become lost within it. Eddie saves them by spraying his asthma inhaler into It's mouth, believing through willed imagination that it is acid, which grievously wounds the creature. In retaliation, It bites off Eddie's arm, and Eddie dies from the injury. Enraged and grieving, Bill, Richie, and Ben pursue the wounded It to its egg chamber; Ben stays behind to destroy Its eggs while Bill and Richie press on. Bill manages to wound It's heart, and despite It's attempt to bargain for its life, the Losers together beat, stomp, and tear the creature apart, finally killing it.

As It dies, the psychic scars on the Losers' palms from their childhood blood oath vanish, signaling the ordeal is truly over. Simultaneously, the worst storm in Maine's history rips through Derry, causing the downtown area to collapse and effectively marking the town's decline. The survivors try to bring out Eddie's and Audra's bodies but are forced to leave Eddie's behind in the ruins. As they leave Derry, each of the Losers begins to lose their memories of It, of Derry, and eventually of each other, just as happened after their first childhood battle. Mike, who had kept the most complete record of events, feels his own memories and even his written notes fading, and finds relief in this forgetting, considering a fresh start elsewhere.

In the aftermath, Ben and Beverly leave Derry together, having fallen in love, and eventually become a couple. Richie returns to his life in California. Bill is the last to leave; before departing, he takes his catatonic wife Audra for a ride on his childhood bicycle, Silver, which somehow awakens her from her catatonic state. The novel ends with the two of them sharing a kiss, as the horror of It fades from memory like a dream, and Derry itself is left changed and diminished by the destruction of the ancient evil that had haunted it for centuries.