“Grotto” tells the tale of Mason Buckley as he joins a new acquaintance on an adventure into the depths of a labyrinthian cave system. His acquaintance, a young woman named Katie, is an aspiring photographer who is teaching herself to “paint with light.” Where better to paint with light than somewhere that light cannot naturally reach? To access these locations, Katie has joined The Bluegrass Grotto, a caving club based in Kentucky. Strangely drawn to Katie, more curiosity than attraction, Mason agrees to accompany her on a caving trip with several other grotto members to help transport and operate her lights. There is something off about the cave, however, and the grotto members seem a bit too eager for new membership. Katie seems to be acting strangely, as well. Or maybe it’s all in his head. The dark can play games with your mind sometimes.