The game is set in an original universe divorced from the Star Wars mythology; it takes place less than 150 years into our own future, and in our own galaxy, rather than one far, far away. Players take on the role of John Shepherd, commander of the Earth Alliance starship Normandy, as he investigates an attack on a human colony world by intelligent machines. As the game unfolds, he must deal with an alien hegemony suspicious of the upstart humans and skeptical of their claims of rogue AIs while simultaneously tracking the machines and their leader through a vast interstellar frontier.

The role-playing component of Mass Effect should be familiar to fans of the genre: There's a main quest that players investigate in order to move the story along, as well as myriad subplots kicked off by major and minor nonplayer characters. As in Knights of the Old Republic, the quests take players to numerous worlds in search of clues and challenges, though this time around the sheer number blows away anything that's come before. There are dozens of planets to visit as part of the regular storyline, but players can choose to go bushwhacking and explore uncharted worlds instead, returning to the main story at their leisure.

While KOTOR used a Dungeons & Dragons-like system of abilities, classes, skills and feats, Mass Effect has its own streamlined system that solves the same problems from a slightly different direction. Players choose from classes, such as soldier or engineer, which grant them access to certain skills. Buying ranks in these skills unlocks new abilities, such as special moves for particular weapons or bonuses to combat. Different classes can give access to biotics, which use game's namesake "mass effect" to throw, lift and pummel enemies from afar.

Combat in the game breaks from the turn-based legacy of most RPGs in favor of something ripped from the screens of Ghost Recon or Gears of War. Players command a squad of three characters: Shepherd plus two NPCs picked at the start of a given mission. Players have direct control over the main character, running and gunning with him using the third-person, over-the-shoulder perspective. A zoom key lets them focus in on targets with an intensity reminiscent of a first-person shooter.

Players can give orders to the two NPCs, directing them to move to a given location or attack a particular enemy, telling them which weapon to use and triggering special abilities such as telekinetic attacks or focused "overkill" bullet barrages. All combat is in real time, but players can pause the fighting to queue up particular actions.

The galaxy is your sandbox

With Mass Effect, Bioware tries to re-create and expand upon KOTOR's winning formula. It succeeds brilliantly.

Mass Effect looks and feels like a game designed for a next-generation platform, taking full advantage of the 360's power to offer incredibly lifelike facial features and body movements that leap to the other side of the uncanny valley. Elder Scrolls IV: Oblivion, an early RPG for the Xbox 360, tried to do the same thing but failed; its characters seemed animatronic in their interactions. With Mass Effect, characters have a body language that players will react to on a visceral level; characters sigh, smile and gesture just as their human counterparts would.

The third-person shooter-style combat provides welcome relief from time spent wandering cities and planets talking to people. The duck-and-cover mechanic echoes that of Gears of War, but it's not as successful. It's all too easy to get hung up on a wall or structure when trying to hide in a firefight, which can be infuriating when it leads to the death of the adventuring party. This is largely because the auto-save function is infrequent and unpredictable; unlike Oblivion and other RPGs, it saves only during major story updates. Players must remember to save often or risk replaying significant chunks of the game.

Another welcome break is the Mako, a six-wheeled tank that's used to explore worlds and wage war. The tank has a learning curve, but once players figure it out they'll have a blast roaming and jumping their way across the landscape. Combined with the run-and-gun style of combat, the Mako goes a long way toward alleviating the fatigue that inevitably crops up playing a game as big as Mass Effect.

Fans of hard SF will appreciate the game's science-based approach to technology. For example, the Normandy has a cloaking device; it redirects all of the ship's heat and radiation into sinks for a few hours at a time. This renders it undetectable to sensors, but the ship itself is still visible to the naked eye. Likewise, the game doesn't assume all planets are friendly to human life; some have fauna that send unprotected explorers into anaphylactic shock.

Mass Effect does a good job of managing the potentially overwhelming number of quests, information, weapons and upgrades it spawns along the way, but it's still a huge game. That's ultimately its greatest triumph—or flaw. Players who love expansive RPGs will revel in Mass Effect's attention to detail and rich universe, but those who don't will likely find it overwhelming and tiresome, even with the addition of new combat and exploration play styles.