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Background

At some point after completing the Reaper IFF mission, when you activate the galaxy map, you'll leave the Normandy with your crewmates, and the Normandy will get ambushed by the Collectors. If you've been thorough, then the Collector attack will come one mission after the Reaper IFF mission (giving you a chance to complete Legion's loyalty mission first). Otherwise, you might be allowed to complete several missions first, to guarantee that you have a large enough crew with more than a few loyal members.

During the Collector attack, you'll be in control of Joker. You won't have to do much during this sequence. You'll just need to follow the "emergency floor lighting" and crawl through some maintenance shafts, plus witness the carnage (including Yeoman Chambers getting dragged off by Collectors). At the end of the sequence, EDI will vent the Collectors into space, but everybody else other than Joker will be killed or captured.

When you return to the ship, you'll learn from EDI that the Reaper IFF is now operational, and that you can travel through the Omega-4 relay at any time. When you go through the relay, you'll have to navigate through a debris field and fend off a pair of oculus attacks in the cargo hold, but you'll end up crashing into the Collector base, where you'll start the mission.

Note: While we're treating this as a single mission here, it is actually considered to be two missions by the game: "Collector Base: Infiltration" and "Collector Base: The Long Walk." The game also calls this mission the "suicide mission" since several of your crewmates can be killed during it.


No One Left Behind

One of achievements for the game is to complete this mission without any of your crewmates getting killed. There are a few things that you'll have to do to achieve this:

• Upgrade the Normandy as much as possible. Specifically, you'll need to purchase the upgrades for Heavy Ship Armor, Multicore Shielding, and Thanix Cannon. Each upgrade you don't purchase will cost you a crewmate during the mission.
  
  
• Go through the Omega-4 relay as soon as you're able. The longer you wait, the more captured crew members from the Normandy (like Chambers and Donnelly) will get liquefied by the Collectors.
  
  
• Gain the loyalty of your crewmates. Loyal crewmates will survive in situations where other crewmates won't.
  
  
• Choose the right people for the job. During the mission, you'll have to select different crewmates for different tasks, and you'll have an easier time surviving if you pick the right people. Tali is a good tech specialist, Jack is a good biotics expert, Garrus is a good leader, and any loyal crewmate can be used for the escort.

Note: While you'll certainly want to complete the mission without taking any losses, it's also sort of fun to play the mission taking as many losses as possible. Most of the cut scenes will change when people die.


Walkthrough

When you crash land at the Collector base, you'll have to divide up your squad. You'll need two "fire teams" to converge at a doorway, and you'll need one character to sneak through a ventilation shaft and deal with the base's security systems. You'll be in charge of the first fire team, but you'll need to pick a character to go through the ventilation shaft, and another character to lead the second fire team. Tali (or Legion) is a good choice for the former, and Garrus (or Jacob) is a good choice for the latter.

Early on in the base, the character in the ventilation shaft will report that there is a gate in the way, and you'll then have to open a valve (aka a "heat exchanger") to prevent that character from burning up. The first valve will introduce you to the concept, but after that you'll be timed, and you'll have to defeat several Collector drones (plus the occasional Harbinger or assassin) to get to each valve. You'll always have about 60 seconds to get to the valves, and that should be plenty of time as long as you keep moving forward. If you don't make it to a valve in time, then you'll fail the mission and have to load a saved game.

Once you've opened eight valves, the character in the ventilation shaft will hack open the doorway, and you and your crewmates will arrive in the main part of the base. There you'll find numerous pods containing colonists and the captured crew from the Normandy, and you'll also learn that the path ahead is filled with seeker swarms.

So once again you'll have to divide up your squad. One character will have to maintain a biotic field to keep the seeker swarms at bay so you can lead a team through them, while another character will lead a diversionary team through a parallel path. You might also want to select a character to escort the rescued crew members and colonists back to the Normandy (if you don't then they'll die). Jack (or Samara) is a good choice to maintain the biotic field, Garrus (or Jacob) is still a good choice to lead the secondary team, and any loyal crewmate is fine for the escort.

When the mission starts up again, you'll be with your chosen biotic moving through the main part of the Collector base. The biotic will maintain a region of safety. If you go outside that region, then the seeker swarms will damage you, but that damage rate will be low enough that you'll be able to run outside for short periods of time (for example, to grab some ammunition) if you really want to.

The biotic will lead you from battle to battle. You'll face almost all of the Collector enemies during this part of the mission, including husks, abominations, scions, drones, harbingers, and more. At the end of every battle, you'll need to talk to the biotic to get your party moving again.

At the end of the sequence, your squad will join forces again, and EDI will recommend that you take a platform to the main control console, where you should be able to destroy the base. However, you'll also learn that the Collectors are closing in on your position, and so you'll have to divide up your squad again. You'll be in charge of the team heading for the control console. Everybody else will stay behind to delay the Collectors for as long as possible.

When the mission starts up again, you'll find yourself on a platform, and you'll have to defeat enemies from other platforms, including drones, harbingers, abominations, and scions. After the battle, you'll find a console on one of the platforms, and activating it will take you to the center of the base.

When you reach the center of the base, you'll find a "human Reaper" being assembled there, and you'll learn that the Reaper is the reason the Collectors were kidnapping humans -- as "food" to construct it. Then more Collectors will show up and attack you.

The Collectors will attack in waves, and during these waves you'll face drones, assassins, harbingers, and more. After each wave, the tubes feeding and supporting the Reaper will become exposed, allowing you to shoot them. If you're quick, then you'll be able to take out two tubes at a time, and finish the battle after two waves. Otherwise, you'll have to fight extra waves.

Once you've destroyed all four tubes, the Reaper will crash to the ground, and you'll begin preparations to destroy the base. However, at this point the Illusive Man will contact you, and he'll ask you to use a radiation pulse to kill all of the Collectors on board, and then turn over the base to Cerberus. So you'll have to make a decision. Should you destroy the base and the horror that it represents, or attempt to mine it for information and use it against the Reapers? This decision won't have any impact on the remainder of this campaign, but no doubt it will make a huge difference in Mass Effect 3.

After setting up your radiation pulse or system overload, you'll discover that the Reaper isn't quite dead yet. It will hoist itself up onto the platform that you're on, and it'll begin attacking you with particle bursts from its mouth. You might also have to deal with a few drones and harbingers.

You'll find that the Reaper is only vulnerable in three places: its two eye sockets, and its chest (both glowing orange-red). The Reaper probably won't climb high enough to allow you to target its chest very often, so focus on its eyes. You'll know you're doing damage to it if you see blue energy bursting from where you're targeting it (and of course if you see its health decreasing). Heavy weapons are the best choice for the Reaper, but you should be able to kill it with anything. Just hide behind crates until you can take a clear shot, and then wear it down.

Note: If you bring the M-920 Cain with you, and if you upgraded Heavy Weapon Ammo all six times, and if you're wearing Ordnance Packs on your legs, then you'll be able to fire the Cain at the Reaper twice, and knock out about 90% of its health.

When you kill the Reaper, the main part of the campaign will come to an end, but you'll be returned to the Normandy, and you'll be allowed to continue completing assignments and missions. Congratulations -- or not, given what shows up at the end of the victory cut scene. "You have failed. We will find another way."