From Vault Wiki

Set 200 years in the future in an epic universe, Mass Effect places gamers in a vast galactic community in danger of being conquered by a legendary agent gone rogue. A spectacular new vision from legendary developers BioWare, Mass Effect challenges players to lead a squad of freedom fighters as they struggle against threatening armies to restore peace in the land.

Mass Effect is the first in a trilogy of games with an overarching story. As the first human Specter – sworn defenders of galactic peace – your mission is to halt the advancing armies of a legendary agent gone rogue. But as you lead your elite team across hostile alien worlds, you will discover the true threat is far greater than anyone imagined.