After their first group encounter with Pennywise in the house on Neibolt Street, the Losers Club is disbanded temporarily with each member going their separate ways and vowing to ignore Pennywise until he disappears again on his usual schedule (for 27 more years).
Unfortunately for them, the clown has different ideas and comes for Beverly after she has a final confrontation with her abusive father. Leaving him unconscious after thumping him with the top of the toilet cistern when he tries to rape her, Bev turns to find Pennywise, drawn by her fear, who abducts her and takes her to his lair.
Drawn back together by her kidnapping, the Losers Club heads into the sewer through the house, where they find It as well as all of his past victims, and are each terrorised by their specific fears, with Stan almost eaten. They discover Beverly, catatonic and floating, and Ben restores her with a kiss and the Club fight Pennywise, overcoming their fears to rob the creature of its powers and attacking it into submission.
Seemingly defeated, It throws itself down into a sub-sewer, the kids escape and the Losers Club is disbanded, with each vowing to come back in the event that Pennywise returns in 27 years (as Bev foresaw in a vision while under It's spell). They make a blood oath, Bev leaves to live with her aunt in Portland and she and Bill share a kiss.