Bring Down the Sky is the first downloadable content (DLC) for Mass Effect. The access to its plot is allowed from the commander's star map on the Normandy after Shepard becomes a spectre. When a massive asteroid threatens to Terra Nova a planet in the Asgard system, you, as Shepard, must investigate origin of the threat.
This add-on introduces the Batarians, a new race of four-eyed bipeds. A Batarian extremist group has hijacked a mobile asteroid station, setting it on a collision course with a nearby colony world.
As in most of the planet exploration missions, you'll start in the Mako infantry vehicle. After eliminating enemy turrets and examining the surface, you'll descend into several bunkers to eliminate enemies, meet survivors, collect items, and find the answers. Two side-quests are also present along with a main plot. In the end of the main plot, Shepard must solve an uneasy dilemma about life and death.
Completing the mission, you'll be awarded with a new achievement, and new weapons/armor will be given to your crew. It is advantageous to complete this add-on's mission during the main Mass Effect gameplay for the additional amount of experience and useful items it provides.