It  is another lengthy novel King [1,138 pages], which makes sense because it actually contains two books in one.
The first part of the story chronicles events that take place during 1957-1958 in fictional Derry, Maine, where a group of misfit kids who call themselves the "Loser's Club" confront a supernatural being in the guise of a frightening clown who preys upon the town's children every thirty years. After several Derry children are gruesomely attacked, the children track the creature to its base of operations in the extensive sewer tunnels underneath Derry, and manage to drive the creature off, ending It's murderous rampage. Afterwards, the children mysteriously forget what transpired, and go on to live separate lives away from Derry.
Thirty years later, the Loser's Club are summoned back to Derry by Mike Hanlon, the only member of the group still living there. Hanlon informs the group that It has returned, and the cycle of horrors is beginning all over again. Regaining their memories of the frightening encounter in 1958, the second half of the book details the events leading up to the final confrontation between the now adult members of the Loser's Club and the monster from their nightmares that is hell bent on exacting revenge.
Given the length of this novel, It was unsuitable to be made into a feature film. Instead, King adapted it for the small screen in 1990, delivering a largely uninspired two-part movie [split over the two timeframes covered in the book] that was most memorable for it's frightening portrayal of Pennywise the Clown.