Mass Effect 2 is "On the House" So if you have Mass Effect 1 but never bothered to play 2, now is your chance. Don't worry, If you have never played Mass Effect before, then the "default timeline" Shepard is still pretty bitchin. WIthout going into too much
Having saved the Galaxy from Saren and the Geth, Commander Shepard is sent to the furthest reaches of the galaxy to investigate a geth sighting when all hell breaks loose. An alien vessel destroys the Normandy Including Commander Shepard as He/She heroically saves Joker who refused to abandon ship. Shepards body is recovered by Cerberus and the Lazarus Project (aptly named) begins to make Shepard Faster, better, stronger etc only be to tasked with saving the universe again (in lay man's terms, Cerberus brought back commander Shepard, it's only fitting to do a few "favors")
Crazy shit happens
So yeah. It also gets all the DLC