Mass Effect allows you to create your own customizable version of Commander Shepard (or jump in and use the pre-created character) and plunge yourself into the center of an epic science-fiction story. Choose your squad-mates, your weapons, skills and abilities, and customize your vehicles, armor and appearance - you are in complete control over your experience.

In the course of your mission to stop the machine invasion, you may choose to follow the path of the soldier, the tech-specialist, or the biotics-specialist…each of which brings an arsenal of unique yet equally powerful abilities to use against the enemy. Wield a multitude of weapon types with precision accuracy…utilize your technical skill to turn enemy war machines against their creators…or unleash the full power of Dark Energy against your foe with devastating effects.
As you progress throughout the game you will improve your character's skills, abilities, and equipment (including weapons, armor, biotic implants and more) to ensure that you have the means to face the growing threat before you.
This customization also extends to the MAKO - the rover with which you will explore the surface of hostile alien worlds. You can improve your vehicle's grip, speed, armor, boosters, weaponry and more.
The role you choose to play in Mass Effect will have tremendous consequences on the galaxy around you. You will face moral dilemmas in which the decision you ultimately make will significantly alter the fate of civilized life in the galaxy.
Your mission will take you into the deepest reaches of known space - and beyond. Aboard your ship, the Normandy, you will choose which planets to investigate, and where to take your team. In addition to the major worlds of the main story arc, you will have the freedom to visit a wide array of uncharted planets and other locations such as asteroids and abandoned space freighters adrift in space. On many worlds, take your team planet side in the MAKO in order to explore and find alien life, ancient artifacts and ruins, rogue trader colonies and a host of other amazing discoveries.

Mass Effect combines astonishing photo-realistic graphics with innovative new dialog systems to create a cast of living characters to interact with. Engage these characters in real-time conversations that allow you to immerse yourself in dramatically charged situations.
Characters communicate with full voice-acting and amazing animation that displays their emotions right down to subtle nuances in their facial expressions. Every wrinkled brow and slight twist of the mouth is captured to infuse every interaction with a feeling of realism.

The struggle to stop the return of the machines will ultimately be a violent one. You will assume responsibility for the lives of your squad as you lead them into often uncharted, hostile alien environments. Each decision you make could mean the difference between victory and disaster. Although combat will take place in real-time, you will have the ability to pause combat and issue commands to your squad which they will then execute with the precision of a highly trained elite force. Upgrading certain skills and abilities as you level up your character will also allow you to increase your ballistic skill - this can ultimately lead to easier aiming, more damaging use of weapons, and biotic effects with the power to turn the tide of battle in your favor.
Mass Effect is an action role-playing game under development by BioWare exclusively for the Xbox 360. It is scheduled for release in Q1 2007[1] and it will be the first installment of a planned trilogy. The game takes place in the year 2183, the player character an elite human soldier named Commander Shepard. The story revolves around an ancient alien prophecy stating that, every fifty thousand years, machines come to the galaxy to harvest all organic life, and the time of their return is approaching. As the first human Spectre – sworn defenders of galactic peace – Shepard's mission is to halt the advancing armies of a legendary agent gone rogue. As Shepard leads his elite team across hostile alien worlds, he will discover the true threat is far greater than anyone imagined.

One of Mass Effect's unique gameplay elements involves its conversation system. Previous Bioware console titles such as Star Wars: Knights of the Old Republic and Jade Empire employed a conversation system where the player chose from several responses after non-player characters had finished speaking. Mass Effect introduces a new system in which responses to non-player characters are displayed and can be chosen from before the non-player character has finished speaking. This allows for more fluid and natural conversations, which gives character interaction a cinematic quality.
In response to a query by 1UP.com, Project Manager Casey Hudson explained the term 'Mass Effect' as follows:
"Inside the game universe, mass effect is a newly-discovered (for humans anyway) physics phenomenon that has properties along the lines of other physics forces such as gravity and electromagnetism. It's what physicists in real life are currently calling "dark energy," as an explanation for the accelerating expansion of the universe -- which has only recently been discovered and flies in the face of the previous notion that the universe's expansion should be slowing down because of gravity."
Thus, mass effect is possibly akin to the theorized fifth force.
In the same article, Hudson explains that certain beings are able to sense and manipulate mass effect much like some real-life creatures (such as sharks and eels) can sense and manipulate electromagnatism. These abilities will be augmentable via implants, and the manipulation of mass effect is referred to as 'Biotics'.
Bioware has released information on two of the races that will play a major role in Mass Effect.
"The turians were the last of the Citadel races to join the Council. Their features are avian, making them resemble humanoid birds or raptors. They have a reputation for skill and bravery in combat, but they are not known to be bloodthirsty. A rigid code of honor and strict discipline are the hallmarks of any turian officer. This includes humane treatment of prisoners and conquered enemies. A turian patrol unit will never willingly leave behind one of their own, no matter what the cost of saving them.
"Turian society is highly regimented and very organized, and the species is known for its strict discipline and work ethic. Turians are willing to do what needs to be done, and they always follow through. They are not easily spurred to violence, but when conflict is inevitable, they only understand a concept of "total war." They do not believe in skirmishes or small scale battles; they use massive fleets and numbers to defeat an adversary so completely that they remove any threat of having to fight the same opponent more than once. They do not exterminate their enemy, but so completely devastate their military that the enemy has no choice but to become a colony of the turians."
"Other species see them as "men of action," and they are generally regarded as the most progressive of the Citadel races. Since their culture is based on the structure of a military hierarchy, changes and advances accepted by the leadership are quickly adopted by the rest of society with minimal resistance."
"The geth are a bi-pedal, humanoid race of networked AIs that resides in the Terminus Systems. The geth were created nearly 300 years ago by the quarians as laborers and tools of war. When the geth began to question their masters, the quarians attempted to exterminate them. The geth won the resulting war. The example of the geth has led to legal, systematic repression of artificial intelligences in galactic society."
"The geth can learn and grow intellectually, but they progress far more slowly than an organic being. Still, the story of the geth's creation and evolution serves as a warning to the rest of the galaxy of the potential dangers of Artificial Intelligence."
"The closer geth physically are to each other, the more intelligent each one becomes. Effectively, they "share" brain power. An individual geth has only a basic intelligence on par with animal instincts, but in groups they can reason, analyze situations, and use tactics as well as any of the organic races."
"Over time the geth have evolved into numerous sub-forms - from the diminutive but highly agile hoppers, to the gigantic, lumbering geth armatures. It should be stressed, however, that in all forms the geth are to be approached with extreme caution as they are universally violent creatures."
Humanity discovers a small cache of highly advanced alien technology hidden deep beneath the surface of Mars. Building on the remnants of this long extinct race - known as the Protheans - humanity quickly masters the science of mass effect fields, leading to the development of faster than light travel.
Spreading out through their own solar system, humanity discovers that Charon, Pluto's moon, is actually a massive piece of dormant Prothean technology - a mass relay - encased in ice.
Once activated, humanity discovers that the mass relay allows instantaneous travel across thousands of light years to a synchronized mass relay in another part of the galaxy.
There they discover several more dormant relays. Over the next decade humanity expands rapidly, establishing colonies and activating dormant relays to open up more and more unexplored regions of space.
To defend its rapidly expanding empire, humanity assembles a massive fleet and constructs an enormous military space station at the nexus of several key mass relays...even though they have yet to encounter another intelligent space-faring species.
Humanity makes first contact with another space-faring culture: the turians. Unfortunately, the encounter is far from peaceful. Over the next several months a brief but tense conflict known on Earth as the First Contact war ensues.
This conflict draws the attention of the Citadel Council - a multi-species government body that maintains peace and stability throughout the known galaxy. The Council intervenes before hostilities escalate further, revealing the existence of the greater galactic community to humanity and brokering a peace between them and the turians.
Humanity continues to expand, founding more colonies and establishing trade alliances with many of the other species who recognize the authority of the Citadel Council. In 2165 the Council makes official recognition of humanity's growing power and influence in the galactic community. Humanity is granted an embassy on the Citadel, the political and economic heart of the galaxy.
Commander Shepard - a promising young officer in the Human Alliance military - is assigned to the crew of the Normandy, the most advanced prototype vessel ever designed.