One month after the devastating geth attack on the Citadel, humanity seized political control of the galaxy.
Now the human-led Council is forced to respond to evidence that the reapers - enormous machines that eradicate all advanced civilizations every 50,000 years - have returned. To quell the rumours, the Council has sent Commander Shepard and the Normandy to wipe out the last pockets of geth resistance. Officially, they blame the invasion on the geth and their leader, a rogue Spectre. But for those who know the truth, the search for answers is just beginning...
The recently deceased Shepard finds himself embroiled in a bid to save a seedy district of Omega to consolidate the help of a salarian professor who may be able to shed some light on the collectors. Favour for a favour, the Commander questions the selflessness of residents of the Terminus systems.
The salarian professor Mordin, old friend Garrus, and psychotic biotic Jack on-board, the acquisitions seem to be progressing flawlessly. Next on the list is the krogan warlord Okeer, who has recently been spotted trading with the collectors. But without readable intentions, Shepard is hard placed to figure out which side the warlord is on.
Disgruntled from his run-in with old flame Ashley on Horizon, Shepard goes in search of another old friend who can help him with the Collectors. Unfortunately, an all too familiar enemy is lying in wait for him.
Amid appeasing a rowdy squad, Shepard receives word that the collector ship they have been preparing to go into battle against is now dead in the water after a turian assault. While they still have the opportunity, the Normandy sets course to inspect the bloodied behemoth and gain some more insight into the collectors' plans.
The crew bickers as Shepard contemplates what to do with the geth platform they recovered from the derelict reaper. Meanwhile, Tali receives word from the migrant fleet and approaches the commander with some not-so-good news.
Reeling after the crew's abduction, the Normandy pursues the collector kidnappers through the Omega 4 relay, despite being wholly unprepared for their final suicide mission. As they depart known space, Shepard reflects on the person he's leaving behind.