The first trailer for Lifetime's You is here, and it brings Pretty Little Liars star Shay Mitchell back to her sleuthing ways. Only this time, it's hard to tell whether she's a victim or a perpetrator.

Based on Caroline Kepnes's 2014 novel of the same name, You explores the dark side of the internet and social media. After Joe (Penn Badgley) saves Beck (Elizabeth Lail) from falling off a subway platform, the two recognize a spark between them and a romance begins falling into place.

Of course, the haunting rendition of "I Want You To Want Me" playing in the background of the trailer indicates this series is more of a thriller than a love story, and things escalate very quickly.

Beck introduces her new beau to her best friend Peach (Mitchell), who senses an air of mystery clouding Joe, and not in a good way. The trailer shows flashes of his obsessive cyberstalking of Beck and his real-life stalking of Peach, and gives a glimpse of Peach's unsettlingly overprotective eye on her friend.

Things get even more intense when both Peach and Joe are seen with weapons in hand — the former with a gun and the latter with a hammer — while Beck fails to grasp how heated this triangle has become. As Joe says, "The things you do for love, right?"

John Stamos also appears in You, which premieres on September 9 on Lifetime.