The Mass Effect games are all about Commander Shepard's quest to become a Systems Alliance hero and save the galaxy from all threats, from Saren Arterius the Spectre to the Reapers themselves. In 2183, Commander Shepard won a great victory against Sovereign and Saren, but another foe was ready to strike while Shepard was still doing a victory lap: the insidious Collectors.
The Collectors serve as the major antagonists of Mass Effect 2, set in 2185, and Shepard reluctantly teamed up with the shady Illusive Man to uncover the Collectors' secrets and take the fight to them. However, the Collectors struck the first blow, and the galaxy was forever changed.
The Collectors Ambush the Hero
The Collectors are no ordinary aliens; they are husk remnants of the once-proud Prothean people, and these insectile cyborgs are utterly loyal to their Reaper masters, Harbinger in particular. For years, the Collectors covertly cruised around the galaxy, obtaining unusual specimens of various species (hence their nickname). They were aware of Sovereign's defeat during the battle of the Citadel in 2183, but they weren't angry; the Reapers have no need for such emotions. Instead, Harbinger recognized Commander Shepard and the SSV Normandy as serious threats to the Reaper cause, and sent its Collector minions to take care of things.
Harbinger intended to learn how an ordinary human aboard a mere frigate could defeat the mighty Sovereign and halt the Reaper invasion, as well as prevent Commander Shepard from dealing another blow to the Reapers. While the Normandy was cruising the Terminus Systems to find Geth resistance, the Collector battlecruiser intercepted the unsuspecting frigate, locking onto it despite the Normandy's stealth systems being online.
Joker, the pilot, was in for the fight of his life as the Collector ship pummeled the Normandy with its Reaper-style beam weapon, and the ship was quickly evacuated. However, Shepard was still aboard, and the Commander forced a stubborn Joker into an escape pod at the last second when it was too late for Shepard to join him.
The Collector cruiser fired another shot, reducing the Normandy to space dust and flinging Shepard into the atmosphere of a nearby planet. Shepard burnt up and died, and their body was later found among the wrecked pieces of the Normandy. But this was just the beginning.
The Struggle for Shepard's Body & the Council's New Stance
Shepard's demise tragic, and it altered the galaxy's course in many ways. The Alliance declared Shepard killed in action, and the Normandy's crew was scattered. Garrus Vakarian ended up on Omega as a tough but well-meaning vigilante, Urdnot Wrex returned home to Tuchanka to begin reuniting the Krogan clans, and Liara T'Soni took it upon herself to prevent any more abuse of Shepard's legacy.
However, Liara's Shadow Broker ally wanted to sell Shepard's remains to the Collectors for profit, and the Collectors wanted a chance to study their greatest enemy in person. Liara turned on the Broker and swiped back Shepard's body, despite the Broker having thuggish Blue Suns mercenaries on hand to oversee the transaction. From there, Liara allowed Miranda Lawson to obtain the body and launch the Lazarus project to revive the Commander. Satisfied with this, Liara journeyed to Ilium to begin a new career.
Shepard's death had another impact on the galaxy: the Council's position on the Reapers and the Geth. In the days after Sovereign's defeat, the Council was completely on Shepard's side and swore to help the Commander face the Reapers when the time came. However, when Shepard died at Collector hands, the Council panicked and reversed their position, denying that the Reapers existed at all.
According to this new stance, Sovereign was a highly advanced Geth dreadnought, and Saren had united the Geth all by himself with his natural charisma. Shepard's death was tragic, but the Council was ready to move on, hopinh to maintain the public's peace of mind by sweeping the idea of Reapers and Collectors under the rug.
Without Shepard, the Council couldn't do much against such threats anyway, so they ignored the problem entirely. By the time Shepard was revived and working alongside Cerberus, the Commander was aghast at the Council's "bury our heads in the sand" plan, prompting the Commander to follow the Illusive Man's lead and take the fight to the Collectors without Council oversight. Shepard could beat death, but not the consequences of that death.