According to their report, the Gareth Edwards-directed/Gary Whitta-scripted film will follow a group of bounty hunters as they work to steal the plans of the first Death Star for the Rebellion. They don’t have much in the way of context for the story, but do include that the main character will first appear in Star Wars: The Force Awakens in December 2015, and that the story somehow involves the Millennium Falcon before Han Solo owns it.
We at Making Star Wars have heard something similar but still don’t know what to make of these rumblings. Not everything lines up with the timeline of the films as we currently understand it, namely the ownership of the Millennium Falcon (which Slashfilm also points out). There are a number of ways it could work (the bounty hunters are unsuccessful in their heist or Han may have just won the Falcon from Lando at the beginning of A New Hope) but until we hear more, it’s best to take it as rumor (albeit, a very intriguing one).