It's the year 1946 and you got a job in SHEILD! What nobody knew is that you had much better training then anyone here. You have been trained in a secret organisation that takes girls and makes them into assassins and spys, kinda like the red room but much more danger, trust me.

You became one of the best assassins but nobody ever knew your face or identity, you maneged to escape the horrible place and decide to start over, and here you are. In SHEILD

But remember! Don't be suspicious when you meet Howard or Peggy! Act Normal!