March 19 marks the day when we finally find out who is the mysterious antagonist of ABC Family’s Pretty Little Liars. After two seasons of  “A” terrorizing our four main characters, that part of the mystery will finally come to its conclusion.

While everyone involved is dead set on keeping us guessing, Keegan Allen (Toby Cavanaugh) tells Wetpaint Entertainment  in an exclusive interview that what you’re about to find out will blow your mind.

“When I found out who that was, my mind was blown because I didn't see how they could do it,” he tells us. “Once you find out who ‘A’ is, you're going to go back through the entire two seasons and be like, ‘Oh my God, how did I not know the entire time?’ Every single episode it's so obvious, but it's so not obvious at the same time.”

Credit: Adam Rose/ABC Family Channel © 2011 Disney ABC Television Group Photo: Toby Makes a Discovery That Blows Spencer’s Mind!

Keegan is convinced it’s going to inspire PLL marathons across the nation, so people can figure out the clues themselves, with the benefit of 20/20 hindsight. And for those who think finding out who “A” is could be a big disappointment after all this buildup, he says the answer is very satisfactory.

“The writers do a really good job of taking this idea and making it realistic because if you really look at the way that this has happened, it seems almost supernatural,” he says.

“They do a great job of explaining it in a way that it's like, ‘Whoa! That's what it is!’ And everyone's going to have their own opinion about it and I think that's great. I can't wait until everybody sees it!”

Keegan’s already back at work on Season 3, which he promises won’t lack a mystery element, despite the big reveal.

“It's still got this element in it of, ‘Okay. I know, but I don't have any idea what could happen next.’ ‘A’ is just one element of hundreds,” he says. “It's a consistent roller coaster of emotions on the show. It's going to be crazy.”

Crazy tense, it seems.

“Everyone is pitted against each other, everyone's lying to everyone,” he reveals. “It's the idea of keeping secrets from your best friend and how that affects the relationship between you. It gets really good, and you're going to keep rooting for your characters and feel very strongly about what happens to them.”

Be sure to tune in for the next episode of Pretty Little Liars tonight on Monday, March 5 at 8 p.m. ET/PT on ABC Family.

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