The shock came in the finale of the series.
About an hour into the two-hour series finale, we get the big, long-awaited reveal.
Mona, wearing the black A hoodie, shows up and knocks Spencer on the head, knocking her out. When she wakes up, she thinks she's looking in a mirror, in some strange room.
But then, her mirror image flinches, taunting her — it's her twin.
Spencer, understandably, freaks out at this.
And finally, her twin starts to explain the story. All the while, Spencer is chained up in some crazy looking facility where she seemingly can't get out.
And, Twist Part II, Spencer's twin... is... British (I guess?). She speaks with an accent in any case. This also explains how they never met each other, because they were on different continents while growing up.
Turns out, Mary Drake had her adopted for a price. She was adopted by a wealthy couple in England, which explains the accent.
The twin explains that she didn't know anything about anyone over in the States, until one day Wren — yes, Wren! — showed up at the bar where she was working as a bartender.
And thus, we get the Twist Part III, that Spencer's twin's name... is... Alex Drake. Which sounds quite an awful lot like AD, huh? So it appears, we have finally learned who is AD.
Alex starts dating Wren and learns about the Rosewood world.
Then, she becomes obsessed with Spencer. She just wanted to be her. After the board game was all wrapped up, she was obsessed.
"I couldn't stop thinking about you, about how could you have it, about how much your friends love you. It was just driving me mad," Alex tells Spencer.
She wanted to "be you, with Wren," but Wren always thought of her as Alex.
So Alex came back to become Spencer, fully, by taking her place in the world. And by hooking up with Toby a lot. Oh, and she also killed Wren along the way. It's all cool.
This has been a pretty well-documented theory, and fans feel vindicated to know their sleuthing was correct.