For many, the main draw of Mass Effect is the detailed and thoughtful story. The overarching story takes place several decades from now, when humans have expanded into space and run into several alien species. We have diplomatic ties to some species, and others are less friendly. Commander Shepard, the main character (players may choose gender and appearance), is on a mission to both bring attention to and defeat the mysterious and dangerous Reapers, who appear to be a threat to the whole galaxy. The Reapers are capable of making corpses-human and alien-into a sort of technological zombie, and these zombies, as well as a bug-like species, mercenaries, and "mechs" (a type of robot) make up the main enemies.
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The history of the galaxy is convoluted and complicated, much like political history is in real life. The Turians and Salarians, for instance, are bitter enemies of the Krogan, a warlike species who were used to fight Turian and Salarian battles. When the Krogan became too powerful and began expanding into other worlds, threatening to take over the entire galaxy, the Salarians manufactured a genetic disease that stopped the Krogan from having children. The Krogan are now a small and destitute species, and many have a deeply ingrained rage toward the Turians and Salarians, who continue to cause their children to be stillborn. The Turians and Salarians are terrified that the Krogan will make good on their word, however, and take over the galaxy once again if they should repopulate.
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Another ongoing struggle is between the Quarians, a nomadic people, and their centuries-old mechanical creations, the Geth. The Quarians, upon encountering a problem with the Geth, decided to put them out of commission-but the Geth rebelled, and kicked the Quarians off of their home planet. The two peoples have been at war ever since, with the Quarians wishing for nothing more than to return home.
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Meanwhile, the mysterious Reapers are threatening to literally wipe out the galaxy. The Reapers appear every few millennia to "restart" everything; unfortunately, no one will believe Shepard in the midst of the ongoing political squabbling. Shepard must gain loyalty from squad mates, fight enemies, and make strategic and diplomatic decisions in order to win the games. He/she may also pursue romance with other characters.
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The themes explored in the Mass Effect series are myriad and complex. The series delves into some extremely heavy thematic material, including, but not limited to, genocide, mental illness, germ warfare, racism, militarism, drug use, and medical experimentation. It also addresses loyalty, love, friendship, self-sacrifice, family, spirituality, and other more positive aspects of life. Most of these ideas are dealt with in a thoughtful and critical way. The game is designed to force the players question their own preconceptions and biases, and it does a good job of it. However, these themes are meant for mature players.