Keegan Allen

Pretty Little Liars fans have not seen the darkest side of Toby Cavanaugh just yet.

On the Season 3 summer finale, viewers were stunned in the final moments when Toby (Keegan Allen) was revealed to be part of the "A" Team. While Toby fooled Spencer — and us — into believing that he was helping Mona and "Red Coat" destroy the girls' lives, he eventually shared that his time as the betrayer was simply to get on the inside so he could protect the love of his life.

Returning summer shows: Where we left off

"I really liked playing that evil side to my character," Allen tells TVGuide.com in the video below. "It actually added dimension, which is something I'm not used to playing on the show because usually he's the moral compass and the perfect boyfriend."

Although the series ventured into a pretty dark place for the Rosewood resident, Allen says that Season 4 will delve even deeper.

Is Alison alive? Is Toby trustworthy? Pretty Little Liars boss on finale burning questions

"Toby goes through a pretty emotional path of his past," Allen says. "I get to finally find out more about him and more about some really insane secrets of his past that are heart-wrenching."

Watch the video for more teases about Season 4, which returns on Tuesday at 8/7c on ABC Family.