The series was very nearly explained in this episode, I think the general rule is that if someone is killed by Viola, the faceless lady in the lake, then they become ghosts who can't leave Bly Manor but if they hold hands with a ghost and say "you, me, us" then that's not entirely true. Basically, when the girl is being dragged into the lake, the au pair saves her by saying "You, me , us" to Viola which saves the girl and is a loving enough act to magically buy the au pair enough time to leave Bly with her lover Jamie. The gardener, Jamie, and the cook live in the village and come in and out of Bly and never die in the first place, it seems.
Jamie and the au pair then meet the cook at a restaurant and he says he says the children now live outside of the manor and remember Bly but don't remember the goings-on, so it seems as if the au pair's self-sacrifice meant that as the children only died by magically holding hands with Viola's ghostly victims, rather than being killed by Viola, they were magically brought back to life, to live outside Bly. Jamie is then invited to a wedding where all the Bly lot are present and tells them everything as a ghost story and they've forgotten everything and ask if Bly is real. This makes no sense for 2 reasons: firstly; why did they invite Jamie to the wedding as they seemingly didn't know her, secondly; why didn't the cook recognise her?
As the living dead au pair became increasingly haunted and possessed by Viola and so found it hard to resist the urge to murder Jamie, she'd left Jamie shortly before the wedding and gone to Bly, sacrificing herself to the lady in the lake. Jamie then went to Bly and swan to the bottom of the lake only to find that the au pair had magically merged with Viola, who was now too selfless to kill Jamie for the ghostly company.
Jamie and the au pair then meet the cook at a restaurant and he says he says the children now live outside of the manor and remember Bly but don't remember the goings-on, so it seems as if the au pair's self-sacrifice meant that as the children only died by magically holding hands with Viola's ghostly victims, rather than being killed by Viola, they were magically brought back to life, to live outside Bly. Jamie is then invited to a wedding where all the Bly lot are present and tells them everything as a ghost story and they've forgotten everything and ask if Bly is real. This makes no sense for 2 reasons: firstly; why did they invite Jamie to the wedding as they seemingly didn't know her, secondly; why didn't the cook recognise her?
As the living dead au pair became increasingly haunted and possessed by Viola and so found it hard to resist the urge to murder Jamie, she'd left Jamie shortly before the wedding and gone to Bly, sacrificing herself to the lady in the lake. Jamie then went to Bly and swan to the bottom of the lake only to find that the au pair had magically merged with Viola, who was now too selfless to kill Jamie for the ghostly company.