After going to Ben's house with Bill, Beverly, Richie, and Stanley, Eddie walks down Neibolt Street, playing some kind of musical instrument, and stops in front of the house; the door opens, and an ominous voice speaks to Eddie. His watch beeps, indicating it is time for him to take his pill. He drops his pills, and as he picks them up, a virulent Leper suddenly attacks and chases him around the house. After outrunning the Leper, Pennywise appears and scares Eddie.
Later, after tracking Pennywise to the Well House, Bill goes there to confront him, though the others try to stop him. After discussing who will go in and who will stay outside to keep a lookout, Bill, Eddie, and Richie go inside, but get separated and attacked by Pennywise.
In August, when Beverly is taken prisoner by Pennywise, Bill, Ben, Richie, Eddie, Stan, and Mike return to the house once more to save her. The six journey into the well, after fighting off a psychotic Henry Bowers, and locate Pennywise's domain, where they find Beverly, and the seven fight against It, defeating him and sending him into early hibernation.
The Well House isn't actually seen until near the climax of the film, when Bill, guilt-ridden over another kid's death at the hands of Pennywise, decides to face It alone. The others remind him that they're all in it together, and with that, they enter the house, ready for battle.
Venturing down to Pennywise's domain, they descend deeper to the very location where It spawned and perform the Ritual of Chüd, to no avail. Pennywise taunts them and takes on a gigantic version of himself with spider-like legs and chases them while trying to paralyze them with its deadlights, managing to do so to Richie, who is saved by Eddie, who gets mortally wounded in the process. After killing Pennywise by mocking him until he shrinks then ripping out his heart and crushing it, the cavern begins to crumble. Bill, Beverly, Richie, Mike, and Ben flee through the cistern and make it out of the Well House as it collapses.
It: Chapter One
It: Chapter Two