No plot summary has been released or leaked yet. Some rumor and hope that they used the Admiral Thrawn series written by Timothy Zahn. But JJ Abrams has stated that this will be an entirely new story. He did however also say in Star Trek that they would not be doing Kahn again, So who knows at this point. All anyone can hope is that at the very least Harrison Ford, Carrie Fischer, Mark Hamill and Billy D Williams get locked into the cast.
In the Admiral Thrawn series, an Imperial Grand Admiral "Thrawn" has taken command of what remains of the Imperial fleet after the Return of the Jedi. As you must realize the entire Imperial fleet was not destroyed at the Battle of Endor.
The story revolves around Thrawn trying to track down another Sith Lord as well as finding a lost fleet of Imperial ships that were running on a prototype crewless automation system. The fleet of ships apparently has been crewless and jumping from point to point in space at light speed for years. No one knows when or where the ships will come out of light speed at. If found the lost fleet would add many ships in good condition to the armada of the empire. In with the story line are the original trilogy characters, years later, with Han and Leia married with children who possess jedi powers. And Luke Skywalker who has started a Jedi Academy.