With most movies and games, sequels give first-installment fans cause to shudder and weep. But every so often, a follow-up can outshine the original. Case in point: Mass Effect 2, which came out in January and sold more than 2 million units in its first week of release. Set in a dystopian year 2185 complete with warlords and gangs, Mass Effect 2 is the wild space frontier that gamers love to imagine. You're back playing the role of Commander Shepard, a paramilitary hero trying to repel a massive forthcoming alien invasion only to be unceremoniously killed at the beginning of the game. Not to worry: you're resurrected with cybernetics and nanotechnology so that you can go on a suicide mission to discover why colonies of humans are disappearing. Depending on how you play the game, you can change the outcome, almost guaranteeing a different story line each time.