For seven seasons, Pretty Little Liars viewers have followed the yellow brick road that will hopefully lead to revealing the identity of "A.D." With a plethora of mysterious characters, one suspect who started the story may likely be the one to finish it.

With more curveballs than one can count, the Marlene King drama has insinuated that twin girls who share a connection with Alison (Sasha Pieterse) are the key to the mystery. While the series has already featured a twin reveal between Mary Drake and Jessica DiLaurentis (Andrea Parker), there have been hints that the sisters weren't the only Rosewood twins — and that Spencer Hastings (Troian Bellisario) herself could have a twin she never knew of.

Ahead of Tuesday's Pretty Little Liars series finale, we propose a theory: What if the mysterious A.D. was none other than Spencer's secret twin? Out of the four girls, Spencer has suffered the most from A.D.'s vendettas and shares the most connections to Alison after learning that Jason (Drew Van Acker) is her half-brother who was the result of her father's affair with Jessica, with Alison's aunt, Mary, her biological mother. From there, Spencer's complicated family tree with Alison has proven to be an important factor in the show's central mystery.

Key scenes throughout the show's history have shed light on a Spencer doppelganger. After being kidnapped by A.D., Hanna (Ashley Benson) dreams of being awakened by Spencer. Visibly missing her bangs, Spencer mentions A.D.'s acronym and sings Hanna the same lullaby that Mary would later sing to her after being shot. A.D.'s existence and Spencer's biological mother were yet to be revealed, which raises the question: How did her friend know that information? It can be assumed that the Spencer that Hanna spoke to wasn't her friend, but a twin.

In the final season, Spencer has occasionally not looked quite like herself. She has a secret meeting with the mysterious doctor Wren and has a noticeable scar on her chest that is different from the gunshot wound she suffered from A.D. The visible differences could hint that the person believed to be Spencer isn't her at all.

Spencer also has a history of blackouts and psychological issues. While a patient at Radley Sanitarium, Spencer had a strange dream that paralleled the A.D. mystery. A girl (Maddie Ziegler) danced around a room, resembling a younger Spencer, afraid and trapped. Neither the dream, nor the relevance to Spencer has been explained, but it can be surmised that the young girl represents someone important to her.

During a conversation that Spencer had with Aria (Lucy Hale) and Mary's doctor, Cochran (Keith Szarabajka), Cochran mentioned that he delivered only two of her babies, hinting that she had more than just two children — Charlotte (Vanessa Ray) and Spencer.

A line from the show's opening credits has always been "Two can keep a secret if one of them is dead." In this case a twin seeking revenge could just be one of the series' finest and most shocking reveals. Was Spencer's twin a rejected child? Did Mary favor Spencer? Until the identity is revealed in the finale, she is just one of the many suspects in Rosewood.

Who do you think A.D. is? Share your thoughts in the comments section below, and stay tuned for more coverage of the series finale. The Pretty Little Liars series finale airs Tuesday, June 27, at 8 p.m. on Freeform.