Spencer Hastings has been throwing herself into finding the person behind Ali's disappearance this season on 'Pretty Little Liars,' and we now know it's all motivated by guilt.

Seems Spencer and the 'PLL' writers have been keeping a very important secret hidden over the course of four whole seasons, a secret which could have direct ramifications for all her relationships.

Read on to find out more, but as per usual, beware of spoilers!

As actress Troian Bellisario revealed in a recent interview about Spencer and Ali's friendship, "With Alison, it was never a cut-and-dry best friendship. Each one of the girls has an incredibly complicated relationship with Alison, bordering on blackmail, and especially Spencer. Spencer was always in competition with Alison."

She continued, "Then, Alison disappeared, and that was a big thing for Spencer to come to terms with. How do you feel when somebody you wished were dead might actually be dead? So, what happens to Spencer is that she thinks she’s the one who has to get [Alison] back because she was the one who pushed her away and didn’t help her that night. She still doesn’t remember what fully went on the night Ali disappeared. So Spencer really takes it on as her responsibility to be the detective and make it safe for Ali to come home."

Troian also discussed a secret that Spencer has been keeping for four seasons, one which we'll get some hints at tonight (Feb. 11) during 'Shadow Play' (episode 4x19).

"Spencer always has a big secret, but this is a big one," Troian teased. "And, this is what’s so great about the show. They are unafraid to keep something like this for almost four seasons. We’re going to learn what actually happened the night Ali disappeared, and it’s going to be so fulfilling for fans who have been waiting years for it to all come together."

She went on to add, "The fans are going to find something out about Spencer this season that the girls didn’t know anything about. It’s going to be a very interesting turn in her life."

Unfortunately for Spencer, this could have a direct correlation with Toby and it may be the end of them. We hope not!

"What really comes between them is Spencer’s secret because as she begins to go further and further down the path of trying to find Alison, she pushes Toby further and further away," Troian explained. "He doesn’t know how to deal with losing her again ... She’s really pulling away and putting Alison before him. She becomes quite obsessed with finding Ali and unmasking ‘A,’ which leaves little room for Toby. So, their relationship goes through a lot this season."

We don't know if we're ready for this, but if you are, you can catch 'Pretty Little Liars' tonight at 8PM ET on ABC Family.

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