You’d think with movies like “Rocky”, “Cinderella Man”, Million Dollar Baby” and “The Fighter”, Hollywood would run out of good boxing stories to tell. Hold on tight. “Southpaw” is a beast of a movie, an emotional roller coaster that threatens to go off the rails, but never does. Director Antoine Fuqua (“Training Day”, “The Equalizer”), working from a solid script by Kurt Sutter, takes you on a hell of a ride. Jake Gyllenhaal, currently on a roll (if you haven’t seen “Nightcrawler” yet, get on it), trained for four months and gained 15 pounds of muscle to play light-heavyweight champ Billy Hope. His wife, Maureen (Rachel McAdams), wants him to slow down and spend more time with his family. The plot truly kicks off when Maureen is involved in a shooting accident sparked by Miguel Escobar (Miguel Gomez), a fighter who thinks he can beat Billy anywhere, anytime. How Billy deals with the situation is spoiler territory that merits careful treading. But Gyllenhaal’s ferocity and feeling pull you in. The brutal boxing action is skillfully staged, and all the actors score knockouts. But it’s the fiercely moving family drama that really matters here, and director Fuqua and his talented crew never lose sight of that. At the end, “Southpaw” sticks with you. It’s a keeper.