The revelation will answer why Spencer was not there when the other girls woke up on the night Ali went missing.
"Pretty Little Liars" are keeping secrets to each other but one of them will have her biggest secret revealed on the show's Halloween special. Troian Bellisario promised this as she attended the red carpet premiere for the said outing.
"I've always been so interested in the night that Ali died. And finally the producers are getting around to explain why when the girls woke up-when Emily, Hanna and Aria woke up-Spencer was not there and Ali was not there and we kind of go in to a secret that the girls didn't even know about Spencer," she said as quoted by "On Air with Ryan Seacrest".
The Spencer depicter added, "It's kind of amazing that now four years in to the show the girls can still have big secrets from each other and not just about their families they didn't know, but truthfully about their history. And so this secret comes to light with Spencer and she struggles with it a lot and tries to keep it a secret and it's all spurred by this possibly of Ali being alive and Spencer falls in to bad habits to try to find her."
To Parade, Bellisario also said that this "secret" would affect Spencer's relationship with Toby. "And this season, with Spencer, there's a really big secret that's going to come out-something she's been hiding from her friends pretty much since they knew Allison. And it's definitely going to throw her plans awry and affect her and Toby to the point where they just can't go back," she further spilled, adding that the couple would be "on-and-off again."
Titled "Grave New World", this year's Halloween special of "PLL" will air Tuesday, October 22 at 8/7c on ABC Family. The episode will see the four girls in Ravenswood after finding a clue in "A's" new lair that Ali may indeed be alive.
It will be followed by the premiere of the show's spin-off "Ravenswood".