Here is a disturbing and ugly story: An older man preys on a 13-year-old girl. The crime is discovered. The man sheds his old life. The girl grows up. And then the narrative goes in directions you would never expect and will revulse you to the core. This is Una, director Benedict Andrews' effectively disturbing film based on David Harrower's 2005 play Blackbird, and it arrives at possibly the best or worst time in the cultural conversation. Dissecting the how's and why's of the narrative can send you down a black hole of infinite darkness, but there is no question that Andrews wrings two devastating performances from his leads, Rooney Mara and Ben Mendelsohn. The latter has become an expert at playing the layered villain, while Mara destroys her title character from inside out with a dedicated level of empathy. These are not easy people to understand, nor to watch unravel, but they are urgent, complicated, captivating characters. Andrews, working from a fresh screenplay by Harrower, almost takes a perverse delight in tearing them apart. You may feel the same once the end credits roll.