When Mason Lockwood strolled into Mystic Falls at the start of Season 2, the Salvatore Brothers were on alert. As if learning he's a werewolf wasn't threatening enough, the last episode of The Vampire Diaries' revealed he's in cahoots with Katherine — everyone's vampire enemy. Taylor Kinney, who plays Mason on the CW series (Thurs, 8/7c), insists that he returned in large part to mentor his nephew Tyler (Michael Trevino). The actor talks about whether he believes his character has fallen under Katherine's spell, and what that moon stone really does.