A motorcycle stunt rider (Ryan Gosling) discovers that the woman he loved and left a year earlier (Eva Mendes) has given birth to a son by him, and his half-baked decision to become a better father ends tragically. This naturalistic drama is ambitious to the point of being unwieldy, and after a while it begins trotting out familiar characters (Ray Liotta as a dirty cop) and situations (an execution in the woods, a la Miller’s Crossing). But once the story has advanced from one generation to the next and its thematic sweep has become apparent, these flaws seem much more tolerable. Derek Cianfrance (Blue Valentine) directed a script he cowrote with Ben Coccio, whose stunning debut feature, Zero Day (2003), deserves a much wider audience.