Eventually, you'll discover that Saren, your vengeful arch nemesis in Mass Effect, has been using the planet as a research facility. He's already gathered an army of virus-like Geth and now hopes to build a deadlier faction of battle-skilled Krogan. Although the Krogan are a slowly dying race due to their inability to impregnate females, Saren has manufactured a cure and begun breeding. Your allies, the Salarians, have discovered this base and request your help in destroying it.
Forget that your head is spinning in a science fiction-fueled haze after that last paragraph. What's important, again, is choice. One of the squad mates you might bring with you on the mission is a Krogan and, although he despises Saren as much as you, he will be severely tempted by the hope that this facility represents to his people. He might stand in your way.
Can you leverage your character's charm and negotiating skills to work through the problem, difficult as it may be? Or will you choose the easy solution, with quicker but more disturbing results? We promised we wouldn't reveal the ending we witnessed, but trust us, the decision you make will be felt for a long, long time. Of course, if you leave the Krogan behind for this particular operation - or never allow him to join your party in the first place (that definitely seemed possible from one of the conversations we heard) - you may not have to make the decision at all.
As in real life, every past choice affects the present and every present choice will affect the available choices of the future. Since BioWare confirmed once again that Mass Effect is the first part of a planned trilogy, your chosen path might even carry over into future adventures. No pressure.
Mass Effect has finally been scheduled for release this November. To see the awesome E3 trailer, check out the next page or the Movies tab above.