It's being developed by BioWare, the studio behind Knights of the Old Republic and Jade Empire. They're describing it as "an epic space saga that spans the entire galaxy".
You take on the role of veteran soldier Commander Shepherd, who is on a mission to stop some other bloke invading everything. The choices you make affect how the game's storyline plays out.
"The best games bring players into an immersive, realistic world that is fun to explore from beginning to end," said BioWare boss Greg Zeschuk.
"We've achieved that in Mass Effect by putting the player at the center of a compelling, cinematic storyline full of realistic characters and hundreds of exciting locations." Confident then.