The "Pretty Little Liars" will solve one of the show's biggest mysteries. Is their bestie, Allison, actually dead? The liars have been trying to get to the bottom of that for four seasons now, and they (as well as the fans) will finally learn the truth on Tuesday night (October 22).

"This whole episode, you know, we follow Red Coat to Ravenswood, which is this really strange town because we think that Ali is there," Lucy Hale revealed to MTV News. "We think that Ali is still alive. And so, at the end of the episode, it is revealed if Ali is in fact alive or dead," she added about the Halloween-themed episode.

"And that sort of sets up a whole new chapter for all the girls' lives because if she is, in fact, alive, it's like everything that they've been searching for is a lie and so there's all these new questions."

While fans learned the identity of "A" at the end of the summer finale back in August, this new tidbit should leave fans wanting more. Hale said, "It just sort of answers a lot of old questions and reveals a lot of new ones. It's like starting a new chapter of 'Pretty Little Liars.' "

Sasha Pietrese, who has played Allison in flashbacks over the course of the series, had one warning for fans. "If you miss this episode, you will miss a lot of the incredible secrets that will unravel during the second half of the fourth season," she teased. "So this is a very, very important episode. It's very fun. It's based in a cemetery."

Showrunner Marlene King mirrored that sentiment. "There's an emotional fallout at the end of the episode. There's a huge revelation that will truly land on these girls in a way that they were never expecting," she said. "They never saw it coming ... that's what fans are going to remember: this final moment of the show where you're like, 'Oh my god!' "

While the twists and turns are sure to keep fans entertained, "Liars" star Troian Bellisario promised that the Victorian-themed Halloween costumes whould also satiate fans' appetites for sartorial fun.

"The costumes are incredible because Mandi Line, our costume designer, is amazing and she poured her heart into this," Bellisario said. "I'm really excited for fans to see [the episode'], especially because I think it will be introducing them to a totally different look and a really great introduction for 'Ravenswood,' which I think will be so cool."

The same night that the liars unravel secrets in the town of Ravenswood, the spin-off series of the same name will premiere on ABC Family. "PLL" airs at 8 p.m. ET with "Ravenswood" following immediately at 9 p.m. ET on ABC Family.

When the Halloween-themed episode wraps up, "Pretty Little Liars" will return on January 7.