Mass Effect was the very start of the trilogy about Commander Shepard in his journey to save the universe from Reapers - an old civilisation that wants to kill every possible rational being in order to prevail any wars. You play as Shepard. With flexible backstory and different classes you travel to Eden Prime with Captain Anderson and Nihlus Kryik, you and your team must discover the mystery behind the attack on the human colony. In the process, it is revealed that another Spectre, Saron, has become rogue and escapes the planet to awoke Reapers and start a full-scale war with the galaxy.
For BioWare, third-person shooters with tactical elements is a very common genre. Four different weapons - automatic rifle, shotgun, pistol and sniper rifle can be obtained through the game with better characteristics. Also, you can level up your protagonist and his squad to gain better stats and new skills. Dialogue panel is simple yet efficient - you choose the style of your approach and build a reputation of a hero or a renegade. With this options in hand, you can build your own story as Commander Shepard in saving the Milky Way Galaxy.