In the past, Nell begins to see the bent-neck lady on her first night at Hill House and the next night as well. 

Adult Nell goes to a sleep technician to discuss her sleep paralysis and the two end up going on a date. A montage shows Nell and Arthur dating, getting engaged and then married. After marriage, he tries to help her during a bout of sleep paralysis and when he gets up to turn on the lights, he drops dead of an apparent brain aneurysm and Nell sees the bent-neck lady once again. 

After Arthur's death, Nell sees a new therapist and slowly begins to act out, by confronting Theo and Steven and also scoring heroin for Luke before taking him to rehab.

After her therapist challenges her to confront her past, Nell travels back to Hill House.

Flashbacks show that Hugh took the kids to a motel after leaving Hill House the final night and came back the next morning covered in blood. 

When adult Nell is back in Hill House, she sees a version of the old Hill House, complete with visions of her family and Arthur. When she gets to the top of the staircase, she imagines her mother putting a locket around her neck when she's really putting a noose around her neck. She then plunges to her death and begins falling through all the many instances she saw the bent-neck lady, revealing herself to be the bent-neck lady.