Mass Effect, the first game of a planned trilogy, follows a segment of Commander [Enter First Name] Shepard's heroic journey. It was developed by Bioware, also responsible for the acclaimed, Star Wars: Knights of the Old Republic. It is an action-RPG set less than 200 years in the future in our familiar universe. There is almost no exposition for the player to familiarize him/herself with the world or experience the Commander's life before things go to hell, but in a video game, the player wants to jump into the action, not control an XO going through his/her daily routine, even if it is in outer space. Luckily, a scrolling paragraph of explanatory text and the crew members of the SSV Normandy are happy to bring you up to speed.
It is the year 2183 AD and humanity has become mostly united under the Systems Alliance in our dealings with the rest of the creatures in the galaxy. Thanks to the technology of an extinct alien race, all of the intelligent species have the ability to warp through space using "mass effect" technology. Since we're still the n00bs of space travel, the other aliens sort of look down on us, but they are apparently secretly afraid of our tenacity and innovation. Meanwhile, many people, including our ambassador, are pissed off that the Citadel Council (made up of the three most prominent species in Citadel Space: the turians, the asari, and the salarians) won't let humans join the Council or the elite defenders of the galaxy: the Spectres. Other species include the giant, monotoned elcor, the short, obese volus, the four-eyed batarians, and the floating, jellyfish called hanar. All of this takes place on the Citadel a massive space station where all the diplomacy in the Milky Way occurs. The politics are an interesting subplot that players can keep in mind or willfully disregard for interesting reactions from the politicians.
You are Commander Shepard, male or female, depending on your preference. You can also choose your basic background which slightly affects certain dialogue and gives you a unique, optional side-quest. It's a nice touch that helps you shape your character a little bit more than previous RPGs, but it does not allow you to project as much of yourself onto the character as KotOR did with it's nameless protagonist's mysterious past. On the whole, Mass Effect is better off for choosing Shepard as its hero because his allies and enemies react to him as though he is an established part of the galaxy and can actually refer to him by name. You can also select the skill Shepard specializes in (though I usually just take soldier because controlling the other powers is not very smoothly integrated with the combat, much like this aside is not smoothly integrated with the rest of my paragraph) and fine tune your facial appearance.
As far as the main plot goes, however, these details don't really matter. On a seemingly routine mission, Shepard finds out that Saren, a Spectre gone insane, is doing his best to bring about the return of "the Reapers," evil, giant robots that have been responsible for the repeated destruction of all organic life in the galaxy and are attempting to do so again. Sounds a bit like The Matrix in space... except for that we can all have hope that the second two parts won't suck. Like any good citizen of the universe, Shepard wants to prevent Saren from achieving this and consistently makes sure that everyone knows it, going on about "stopping Saren" before he even knows what the turian is up to.
After proving to the Citadel Council that Saren is a villain, Shepard is made the first human Spectre while Saren's position is stripped of him. This point in the game is when the main quest really begins, with Commander Shepard chasing leads around the galaxy, visiting planets, killing Saren's minions, and finding clues that will eventually lead him to the final confrontation with the bad guy. If it sounds familiar, that's because it is. Mass Effect borrows a lot of elements from Star Wars: Knights of the Old Republic, including the gathering of party members as you go about completing your quest.
As you travel from planet to planet, nonobligatory tasks are offered to you, sometimes with many lives in jeopardy, sometimes with political power on the line, and sometimes personal relationships with NPCs at stake. The way Shepard (you) goes about his business defines him as either a ruthless or noble Spectre. For example, you can choose to execute a prisoner or let him go (I guess arresting Fist would have been too much work?). In the downloadable content package you can either let the terrorist go and save the hostages or let him blow up the civilians and then torture the bastard to death. It's the choices that have no clear "right" answer that make the freewill system great. Unfortunately there are only a few of them in the game, but when you have to decide which one of your party members to rescue, leaving one to be vaporized by a nuke, it may be a tough decision. Throughout your adventures, you really start caring about the characters and may even form a romantic relationship with one or two. You will feel a range of emotions for your party of core followers: affinity, humor, annoyance, even compassion, since the characters are so well drawn, though I'll go into more detail about them later. 
Aside from the characters, the story in Mass Effect is the main motivation for players to keep playing. You are frequently reminded of the urgency of your task and Shepard seems very set on accomplishing it, whether the player is in a rush or not. This immediate feel to the story keeps the tension high, but it makes the side quests feel like more of a distraction. Would Commander Shepard really stop to counsel an arguing couple on his way to save the galaxy from annihilation? It's up to you and you will likely be pulled in two directions: wanting to do everything and help everyone (or kill everyone), but also wanting to solve the mystery and save/takeover the galaxy. I found that a balance of helpfulness and ignoring some pleas for the greater good made for a satisfying and dramatic story. The NPCs are quite persuasive, though, and I definitely ended up saying yes more than no because their disappointment at being left to fend for themselves is so believable and pitiable.
There are five core planets you must visit before the climax and each has its own atmosphere and secrets. Going through them the first time is interesting, but once you've figured out what to do, the second playthrough is very similar. There is usually a choice right before you finish with the planet to kill something or let it live, but the path to this is linear and could have done with some more freedom. Hoth - I mean Noveria, the ice planet, needs the most work. It is controlled by greedy corporations and a corrupt Administrator. The Administrator gets what he deserves, but the corporation, which is performing illegal research that leads to a massacre by crazed insect-like aliens, gets off scot free. There is no closure on what happened to the surviving scientists in the research labs, either. At least Saren's half-evil henchwoman gets wasted here, which distracts from the injustice of the pricks running "Binary Helix" staying in business. Virmire, a planet with beautiful beaches but no tourists, is very exciting by comparison. The stakes have been raised and several key choices about who in your party survives are made here. There's no real puzzle to solve, but there's a battle to fight. Of course, not every planet could have this level of intensity... there wouldn't be room to tell a story with all the action and you'd run out of party members to sacrifice. For the best dramatic results, go to the planets in this order: Therum (pick up Liara early so you have everyone together for the adventures to come), Feros, Noveria (let it blend in between decent planets), and Virmire (meet Saren face to face for the first time and elevate emotions before the final planet).
The fifth and final world you visit is Ilos, and it's fairly ugly. That doesn't really matter anymore because the chase is coming to an end. Shepard pursues Saren across the planet, finally has all relevant questions answered by an ancient hologram, and then warps back to the Citadel where the evil turian is preparing to unleash the Reapers on the galaxy. After a nifty sequence of Commander Shepard and his allies running up the outside of the Citadel tower in a vacuum, the hero and villain meet for the final fight. Saren flies around on a mount he must have stolen from the "Green Goblin" and attempts to blast Shepard to oblivion but is defeated (as long as you have some skill). He can also be persuaded (a little too easily) to blow his own brains out for the greater good, saving the player the trouble of shooting him. There is a deadly naval battle going on in space outside the station with Saren's robotic ships and Sovereign, a Reaper, fighting the humans and friendly aliens. Shepard can make the call as to whether or not the Citadel Council survives the battle. He is then interrupted by the corpse of Saren, possessed by Sovereign, coming back to life to fight again. This second boss fight is about as hard as the first. With the destruction of the reanimated Saren, Sovereign's shield fails and the Reaper is blown to pieces by the Normandy.
Shepard and his crew are the heroes of the galaxy and if the Council is still alive, they can offer humanity a seat. Otherwise, the human ambassador declares humans the new ruling species. Either way, Shepard can decide who will take charge of humanity, his honorable mentor, Captain Anderson, or the shrewd and merciless Ambassador Udina. Those players who took the "Renegade" path would likely pick Udina, while "Paragons" will naturally nominate Commander Anderson.
One drawback to the Renegade Vs. Paragon system (as opposed to KotOR's Light Side/Dark Side alignment) is that both stories and endings are pretty similar. The bad guy is defeated, humanity is victorious, the galaxy is saved. The only real difference is whether a cruel, racist bastard or a humane, virtuous hero did the saving. You can be a jerk but never really evil. There are some truly hilarious moments of "bad guy" dialogue, such as the right hook to the jaw of an obnoxious journalist, but overall, it's not as fun to be mean when you can't go all the way and see your brutality as a pleasure and not a necessity. Both the good Shepard and the bad Shepard have the same fundamental objective, their methods simply differ. Basically, since it's about as easy to be nice as it is to be nasty, why not be nice? Personally, when I tried to play through the game as a callous scoundrel, I had trouble being as mean as possible because I felt too sorry for the characters, mainly good people, to whom I was being unnecessarily vicious. At least in this regard, the good/bad system is successful. It feels very natural to who the player is as a person and can create a balance between the two extremes. You can't be both a Sith and a Jedi, but you can save millions from a falling asteroid and still quietly execute some punk who will only cause more trouble if you let him go.
In the end, you make your story best by doing what feels right to you. It makes the primary adventure a bit more linear overall because you cannot choose one extreme over another, but the smaller missions, actions, and conversations can be handled differently on each run through the game, making Mass Effect's story the sum of many diverse parts.
