When he wakes up in the morning (seriously, how does the guy keep falling asleep under these conditions?), Sam Daily is at the house.
Arthur explains all the scary things that happened to him, and they pack up his stuff to head back to Crythin Gifford.
In his room at the inn, Arthur looks through the letters, where he finds a death certificate for Nathaniel, Jennet's son, who apparently died in the marsh.
Somehow, this is enough for him to put it together. He checks with Sam Daily, who confirms.
Here's the story: Jennet Humfrye got pregnant when she was unmarried, and her married sister Mrs. Drablow convinced her to give the child up. The Drablows raised the child as their own, and Jennet convinced them to let her live with them so she could see Nathaniel.
One day, when the boy was in the pony and trap with his nursemaid and Keckwick's father, it became stuck in the marsh and they all drowned. (This is obviously the pony and trap that Arthur keeps hearing again and again.)
Jennet was watching this whole time from the nursery window. Seeing her child die before her eyes drove her insane. When she finally died, she kept right on haunting the town.
Daily warns Arthur that every time the Jennet's ghost has been spotted, a child has died in mysterious circumstances.
Arthur is taking all of this in—and freaking out a little—when Stella arrives out of nowhere. Yay, reunion! Is it time for our happy ending now?