‘Tis the next century. While exploring Mars, humans have discovered the ruins of an ancient spacefaring species called protheans. By studying the remains of their technology, they figured out deep-space travel with the help of structures called mass relays. This led them to realise that, not only was a sizeable portion of the galaxy outfitted with those mass relays, thus permitting instantaneous travel from one point to another, but that there was a plethora of other, often more advanced alien species out there–asari, turians and salarians being the three main ones–, organised in a galactic community centred around a huge space station called the Citadel. The nations of Earth eventually unified under one joint command, called the Alliance, and, as the game begins, in 2156, humans are eager to make a name for themselves as one of the major species. However, this entails having a representative elected to the galactic Council, as well as at least one member in the Spectres (basically the galactic Interpol). Considering that humans are generally perceived as upstart newcomers, this doesn’t sit well with many of the other species. Nevertheless, Shepard, a skilled and reputed Alliance Commander, is selected as a Spectre candidate and sent on a supervised mission aboard the Normandy, a prototype Alliance vessel, to a planet called Eden Prime to retrieve a prothean artifact. The mission is designed to let the Council assess her/his abilities, but also to nab some useful prothean technology. However, it all goes horribly wrong, as the planet is attacked by self-aware robots called geth, and soon, it transpires that a rogue Spectre called Saren is trying to unleash an apocalypse on the galaxy by contacting sapient machines called Reapers, which were apparently responsible for the extinction of the protheans, 50,000 years prior to the game’s timeframe. Shepard is made a Spectre, given leave to recruit whoever s/he deems worthy and instructed to put a stop to Saren’s plans.
There’s some first-class space opera stuff going on here, so if that’s your cup of tea, you’re in for a treat. And the Reapers are an original idea for a villain.