When Eddie Byers was one year old, he was kidnapped by a women named Sonia Kaspbrak She was desperate to have a son, the perfect boy. It was an easy kidnap, and she was successful. For 16 years, Eddie 'Kaspbrak' was locked away in a room for his entire life. The room contained a bed, a bathtub, a toilet, and some things to entertain Eddie. Eddie had never left the room, he was always told that the room was his entire world, and that he and Sonia were the only two people left alive.
When Richie Wheeler was two, he was kidnapped by the same women, except Richie knew her as 'Maggie Tozier', a fake name Sonia wanted to give. She wanted another one, unaware of how bizarre Richie was. She kept the two boys separated, by the rooms didn't look the same.
What Sonia didn't realize, was that the two boys vents were connected, the walls were thin. They had been talking to each other for 11 years, not aware of what the other boy even looked like.
Richie realizes something is wrong, he doesn't feel safe anymore. So when he's 16 he offers his bestfriend to find out what's on the other side of that door.