Here's the synopsis, which appeared on the official site of financier Nu Image/Millennium Films.

The Expendables are back and this time it's personal! After Tool (Mickey Rourke), the heart and soul of the Expendables, is brutally murdered on a mission, his comrades swear to avenge him. They're not the only ones who want blood. Tool’s beautiful young and wild daughter Fiona embarks on her own revenge mission, complicating matters when she is captured and ransomed by a ruthless dictator plotting to destroy a resistance movement. Now Barney and the Expendables must risk everything to save her and humanity.

Just as he's not directing but has a lot of input on choosing a new director, Stallone isn't writing the screenplay this time around, but he has had a hand in the story.  The script is written by David Agosto and Ken Kaufman.  Stallone has said publicly that he would like Bruce Willis to expand on his cameo from the first actioner to play a full-on bad guy in the sequel.  He's also angling for more screen time from Arnold Schwarzenegger.  Either way, there will be plenty of sweaty, bloody action to go around when The Expendables 2 on August 17, 2012.