It's about a prince(tony jaa) trying to revenge his parents death.
Just saw the movie yesterday. Don't let the title fool you though, Ong Bak 2 has nothing to do with Ong Bak. I imagine they used it to get more sales. The production values were really high compared to his other films. He showed more maturity in his martial arts. Instead of just sticking to Muay Thai(which is amazing on its on) he uses a bunch of different MA like kung fu and even drunken fist at one point. A must watch imo for any MA fans and especially for Tony Jaa fans.