Netflix Teen TV shows

Her scenes as Sally Draper from Mad Men seem pretty mature.

I've seen the entire MM series 5 or 6
times, Sally and Sabrina are the same
person. Sabrina is a little older is all.
Every time Sabrina is in a scene, all
I see and hear is Sally Draper.

By contrast, the actress who plays
Mary in Downton, a series I've
seen 10 times all the way through,
I never think of Mary when I see her
in her new series. Never ever, two
totally different people.

Can you tell me the central story of
this series, what the point of it is?
It's confusing to me, it jumps all
over the place like the writers are
looking for the story.