Ok, I'm going to lay upon you the completely unbelievable bit of the plot now. There's a Mixed Martial Arts tournament which is for the top sixteen competitors in the world to duke it out for a big cash prize (plausible). Two estranged brothers, neither of whom have ever taken part in a MMA competition get into the competition as wildcard outsiders (implausible). Ignore this. It's macho sub-Rocky bullshit, but this is not where any of the useful stuff in the film is. So just go with it.
Ok, brother number one is an ex-Marine played by Tom Hardy, who rocks up on the doorstep of his recovering alcoholic father, Nick Nolte. The two haven't seen each other in years, since the son and his mother left, because the father was beating their mother while drunk. The mother has since died, and the son has returned, but with no apparent interest in reconciliation. Then he finds out about the tournament, and manages to swing himself an entry by essentially leathering the middleweight US champion in a sparring session. Having done so, he insists his father, who was his high-school wrestling trainer, train him for the tournament.
Brother number two is a physics teacher played by Joel Edgerton, who's about to lose his house because he can't afford the payments (having basically bankrupted himself paying medical bills for his daughter), and so starts doing exhibition fights in parking lots for a little extra money. The school catches on, he's suspended without pay, and all of a sudden, he's forced to make the fighting his main job again (having retired from the sport years ago). Using his old connections, he manages to get an entry into the tournament. He is also estranged from his father (for the same reasons as his brother), but also from his brother (because when his brother and his mother left, he refused to follow them, and stayed to be with his girlfriend.)
Anyway, what this film is really about is the epic amount of rage and pain within Tom Hardy's character, and his inability to allow his family to help him through it. Instead, he is effectively torturing his father, by being around him, but throwing any attempt at reconciliation back in his face. And that's where this film is absolute dynamite, the abusive relationship between Hardy and Nolte. It's filled with epic levels of bitterness and recrimination, and has a level of emotional brutality that eclipses any of the mere girly slaps going on in the MMA ring. Eventually, of course, it comes to a head as the two brothers inevitably meet in the ring, and the physical pain becomes a metaphor for the emotional pain.
Overall, powerful stuff, and something that is absolutely worth digging beneath the surface layer of slappy fighting to get to.