We will start with the past — for the most part everything we learned was true. Mona (Janel Parrish) was the original A. She tormented Ali (Sasha Pieterse), and then the girls after Aria (Lucy Hale) returned from Iceland. The game was stolen from her by Charlotte (Vanessa Ray) when Mona was in Radley. Charlotte, both Ali's and Spencer's (Troian Bellisario) biological half-sister, became A out of revenge. Charlotte resented Ali because she was being raised by the parents who gave Charlotte up. And she hated all of Ali's friends because they had helped bully her friend from camp, Lucas (Brendan Robinson), for years.
What we didn't know is that during the five A-free years, Charlotte found her own tribe in London. She fell in love with Archer Dunhill (Huw Collins). She reconnected with Wren (Julian Morris), who had already broken up with Melissa (Torrey DeVitto) at that point. And she was introduced to her other half sister — Spencer's twin — Alex Drake (also Troian Bellisario). When Charlotte locked Spencer up in the Doll House, she already knew that Spencer was her other sister and their whole complicated family tree.
When Mary Drake (Andrea Parker) pretended to be her twin sister, Jessica DiLaurentis, and hooked up with Peter Hastings (Nolan North), Mary got pregnant with twins. One of those twins was adopted by the Hastings family and named Spencer. However, Mary sold one of the twins to a rich family in London. This family eventually returned their baby to an orphanage. This baby, Alex Drake, grew up without friends or family, and eventually ran away from the orphanage when she was 10.
Years later she met Wren, and the two fell in love. When Wren introduced Charlotte and Alex, the two half-sisters who had been abandoned and mistreated by everyone, finally found family. However, the lure of the game brought Charlotte back to America to continue tormenting the Liars. Eventually the game would lead to Charlotte's death, and Alex would come looking for her. This is when Alex Drake started going by her initials, AD, and tormenting the girls instead.
Originally Alex just wanted to find Charlotte's killer. Yet, the more she tormented the girls, the more she started to become jealous of their friendships, and the life Spencer got to live. Alex realized everything would have been different if she had been picked to live in the Hastings household. She was willing to give everything up in order to have Spencer's life — to be friends with the rest of the Liars, to have a mom and a sister always there, and to have Toby. She started impersonating Spencer, and even kidnapped Spencer in the finale, so she could steal her identity.
However, there is only one Spencer Hastings. The Liars, with the help of Mona and Jenna (Tammin Sursok), were able to catch Alex and save Spencer. Alex is now (and maybe forever) a doll for Mona, the only person who was ever worthy of being the true A.