Mass Effect is the first part of a science fiction RPG trilogy. It is set in a futuristic universe created specifically for the game, with its own history, various alien races, as well as cultural, political, and social background.

The story of the game deals with Commander Shepard, supposedly the first human candidate for the elite group of special agents (the Spectres), who serve the inter-galactic Council, comprised of the oldest and most powerful races in the universe. With or without the help of the Council, Shepard has to stop Saren, a Spectre member who went rogue and tries to exact revenge upon those he was once a part of.

In Mass Effect the player takes control of Commander Shepard, who is fully customizable in the character creation screen; abilities, gender, and physical appearance can be shaped by the player. Both female and male versions of the character are fully voiced.

Combat in the game is action-based, and is similar to tactical squad-based shooters. As in most of BioWare's previous RPGs, the player can pause at any time to issue orders to other squad members. The squad members are AI-dependable, but general commands (such as run forward, take cover, target a specific enemy, etc) can be issued without pausing.

The six character classes in the game are Soldier, Engineer, Adept, Infiltrator, Sentinel, and Vanguard. Soldiers are good with weapons, Engineers can use tech abilities to sabotage enemies' equipment, and Adepts are able to use the disruptive biotic powers. The other three classes are combinations of the first three. Character growth features, beside the usual "level up" system, a skill-based advancement. When the characters gain a level, the player is allowed to distribute points into skills of his/her choice. These skills include weapon proficiency, tech abilities such as destroying enemy shields or hacking robotic enemies, and biotic abilities that manipulate the mass effect field to damage enemies and protect the party. Non-combat abilities for the main character include Charm and Intimidate, which influences conversation choices.

Mass Effect comes with its own morality system. There are two sides of morality in the game - Paragon and Renegade, with Paragon being a more diplomatic, official military courtesy following character, and Renegade being the "ends justify the means", damning everything to hell kind of character. The main quest with its choices and consequences is only a part of the experience - there are several optional planets that offer side-questing and exploration. Pursuing a romantic relationship with a companion is also possible.

The game introduces a slightly tweaked conversation system in which responses to NPC's are displayed and can be chosen before the NPC has finished speaking. This, combined with detailed facial expressions, allows for more fluid and natural conversations.

The PC version of Mass Effect differs in some points from the Xbox 360 release. Besides higher resolution graphics, the mini game about hacking a computer has been changed. Now instead of playing "Simon Says", the player has to get a triangle into the middle of a circle by avoiding the blocks that are constantly moving around in a Frogger-like fashion. The interface has also undergone massive changes. Besides a new quick slot bar in which the player can assign up to eight abilities for quick access to the number keys, the pause menu has been changed to give the player faster and better control over his teammates. It's now also possible to give every teammate individual orders instead of having both do the same thing. Item management has also been simplified to account for the new keyboard/mouse control scheme which also allows for better precision in the shooter-like fights.