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Throughout Mass Effect, you're constantly making choices. Every single decision changes someone's opinion of you, or alters the potential resolution of a scene, or determines whether you're going to have pancakes or French toast dunkers for breakfast. To list them all would take days; instead, here are the biggies that have the most direct impact on Mass Effect 2.
On Virmire, you're confronted by Krogan squadmate Urdnot Wrex over the goal of your mission (to destroy Saren's cure for the Krogan genophage.) The resulting high drama determines whether Wrex lives or dies, and the outcome in turn influences Shepard's visit to the Krogan homeworld in Mass Effect 2.
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In a later incident on Virmire, squadmates Ashley and Kaidan both need rescuing but in opposite directions at the same time. Shepard is put on the spot to choose who lives and who dies. Complicating the matter is that they are both potential love interests, depending on the gender of your Shepard. Whichever one you save will appear in Mass Effect 2, though probably not as a recruitable character.
Finally, Mass Effect has three different endings depending on your decisions in the game's closing hours. If you save the Council, they gratefully welcome humans aboard and you choose Anderson or Udina as a representative. If you let the Council die, the story concludes with a new Council being formed, either all human (the Renegade choice) or with the other races led by a human chairman (the Paragon choice.) In either case, you're given the option to select Anderson or Udina to be the Council chairman. The political landscape and race relations for Mass Effect 2 are thus determined.
The final page of our primer summarizes additional Mass Effect content that's come out since the first game.
