Priority: Rannoch

The Reaper base controlling the geth has been located, but jamming towers prevent conventional orbital bombardment. Land on Rannoch and use a prototype targeting laser to enable the Normandy to destroy the Reaper base.

Acquisition
To acquire the mission, you must first complete either Rannoch: Admiral Koris or Rannoch: Geth Fighter Squadrons to trigger Admiral Raan informing you of the Reaper Base. To start the mission, orbit Rannoch and select the Reaper base.

If one of the aforementioned missions has not been completed, it will become unavailable once this mission is started.

Preparation
Since Tali will be a compulsory squad member for this mission, consider bringing a squad member who can combo with her Sabotage and Energy Drain powers to create Tech Bursts which are devastating to the geth you will be facing. Overload and Disruptor Ammo are particularly favored abilities in this light since they also help against all the shields you'll be encountering. A side-helping of anti-armor abilities will also come in handy since you will be facing several Geth Primes.

This walkthrough assumes that Legion and Tali survived the suicide mission. If Legion died or was given to Cerberus in Mass Effect 2 or the player did not import an ME2 save, its place is filled by a "Geth VI". In addition, if Tali died in your imported ME2 game, you may choose your second squad member freely and Admiral Xen will support you.

Coming in Hot
Upon arrival at the Reaper base, your squad will be attacked by several geth hostiles: five initial Geth Troopers and a second wave of other Geth Troopers. Eliminate them and proceed down the hill. Once you near the bottom, the large blast door will close and three more Geth Troopers will drop down to impede your progress. Make short work of them. Check near the inside-right of the blast door for a SMG High Caliber Barrel upgrade.

Move to the right of the blast door near the tubes and vault up them. Climb the ladder on the right and eliminate the Geth Trooper waiting at the top. Vault onto the pipe and move forward. Eliminate the two Geth Troopers and Geth Rocket Trooper, and move along the right side to their position. Eliminate the Geth Rocket Trooper on the catwalks in front of you. Pick up an Assault Rifle Piercing Mod and Kassa Fabrication Gauntlets along the way before proceeding to climb up to the next level.

Eliminate the geth patrol below you consisting of two Geth Troopers, and find cover. Four Geth Rocket Troopers will drop in, and once they have been eliminated another four Geth Rocket Troopers will siege your position. Once all hostiles have been eliminated, press forward and through the doors. Salvage the Reaper tech on the right for 10,000 Credits.

Fight Your Way In
Proceed up the ramp and destroy the lone Geth Trooper. A blast shield will expand over the Reaper tech broadcasting the signal. At the junction, go to the left to pick up an SMG Ultralight Materials mod and head back towards to other direction. As you move along the linear path, numerous hostiles will attempt to impede your progress. Fight your way through the geth forces: two Geth Rocket Troopers, eighteen Geth Troopers. Near the start of the walkway, look for an Assault Rifle Stability Damper upgrade on your right. Once all hostiles have been eliminated, Legion will ask if you need assistance. Grab the Geth Plasma Shotgun and salvage the data worth 5,000 Credits on the left of the door.

Move through the door and salvage more data for 10,000 Credits and a MedKit. Proceed through a second door and a nav-point will be marked on your heads-up display. Hostile geth will filter in from the left and the right. Eliminate all hostiles before activating the Door Override. Geth will filter in from the left side; once all hostiles have been eliminated, you will be informed that a manual override is needed and will mark a console on your heads-up display with a nav-point. Proceed to the nav-point and activate the Door Override. Legion will inform you that you cannot double back, but you can use the elevator marked on your heads-up display. Move towards the elevator, but be ready. When the elevator lowers, a Geth Prime will be riding it; eliminate it and the two Geth Rocket Troopers that drop down before boarding the elevator. There's a MedKit on the right of the elevator if you need it.

Triple Prime
Ride the elevator up to the next level and be ready for a fight. Three Geth Primes will be standing with their backs to you: one on your left, one on your right, and one straight in front of you. Grab the Geth Spitfire in-front of you - it will come handy against the Geth Primes; in case it runs dry, there will be two others to your left and to your right. Also, it is recommended that you take down the enemies' shields first, after which their armor will stand no chance against the Spitfire.

After eliminating the three Geth Primes proceed to the nav-point. Shepard will pull the laser designator from his or her back. Designate the target by holding down the trigger and making sure that you stay outside of the blast zone, marked by the blue circle. Note that the target is in front and below you, and not where the objective points. Once designated, the Normandy will proceed with its bombing run. The explosion knocks Shepard and their squad off the upper-platform, and a Reaper begins to climb out of the hole.

Legion tells Shepard that it has acquired transport. Sprint to the geth ship. If you don't get there in a decent amount of time, the Reaper will fry you. Shepard will get on top and man the turret while the squad gets inside with Legion piloting.

With the ship on the move, target the Reaper off to the left of the ship while Shepard calls in for an orbital strike. (You don't actually have to target or fire at the Reaper, eventually the vehicle will be far enough and Shepard will yell clear). When the orbital strike hits, the Reaper goes down temporarily while Shepard orders EDI to sync the quarian fleet to the Normandy's targeting systems. Determined to take down the Reaper, Shepard stays back while Legion retreats to a safe distance with Shepard's squad.

Deal With a Reaper, Personally
Move to the center of the plateau and aim at the Reaper. A set of rotating targeting circles will appear. Begin targeting the Reaper by aiming and holding down the trigger. The Reaper's first beam will not be targeted at you, being directed at the sky above, giving you ample time to figure out what to do. If, for some reason, you hold off from firing, Admiral Gerrel will complain about losing the fleet. Soon after, Legion/Geth VI joins him, and soon after that, the game over screen pops up and you'll have to restart.

Once the orbital strike hits, the Reaper will move closer and begins to deploy its beam cannon at you. Be prepared to dodge to either the left or the right. Once the beam has sliced past you, the circles will appear again. Only during that time can you aim your laser pointer and begin to compute a firing solution. You only need to dodge and hit 3 times after the initial strike in order to bring down the Destroyer.

The key to surviving this encounter is to know that, counter-intuitively or not, you don't lose your progress if you have to stop aiming and dodge. Pull up the laser pointer and you'll discover that your firing solution is where you left it and can be resumed. Also note that moving while using the laser pointer causes the target acquisition process to go much slower. The other key is that the Reaper's aim is based on what you are doing when it starts firing.

Note that on Casual, you don't have to do this twice. As soon as the beam touches ground, sprint to the side, turn, and fire. The Reaper will finish the attack, recharge, and try again. It won't make it. By the time the beam reaches you, you'll finish targeting and it will be hit from orbit. You only have to move once for every beam.

On insanity, it feels like the beam is faster, and being even close to the beam insta-kills you. As a result, you'll need to make wider sprints, corner to corner, giving you less time to fire. Same strategy applies, keep running, stand still, fire for eye. Except run further, corner to corner. As soon as Reaper starts firing, sprint, turn, fire. Performing a dive-roll as the last part of your sprints will help you get turned toward the reaper more quickly.

After the initial strike (the one where the Reaper is aiming at the fleet) and three dodge-and-sprint regular strikes, there will be a final super-slow-motion strike where you must stand where you are, aim and fire. Unlike the previous strikes where extremely approximate aim works out fine, this final slo-mo strike specifically emphasizes aiming accuracy. Do not panic and aim just anywhere in/near the target zone - instead take a moment to aim right in the bulls-eye as this is the only way the targeting process will complete before the Reaper beam kills you.

Once you complete the targeting procedure, the game will switch to a cutscene of the quarian fleet firing upon the Reaper. While it is disabled, it will speak to Shepard about Harbinger and the inevitability of Shepard's defeat, but will also allude to a higher power orchestrating the entire process. Depending on what you select as a dialogue, you will get:


 * Paragon (top) choice: Paragon dialogue, about how organics will win. Shepard keeps on organics' side, the Reaper brings up the quarians as an example. A Paragon interrupt will follow. The Reaper simply powers off, as if not wanting to talk any more.


 * Renegade (bottom) choice: Renegade dialogue, about taking down the Reapers. A renegade dialogue will take place, followed by a renegade interrupt. Shepard will target the Reaper one last time, killing it.


 * Ambivalent choice (help us understand): No interrupt will occur, the Reaper will prattle on about cycles of extinction but offer no real insight, only hints. It then dies of its own accord.

Once this is done, Legion/VI will appear in the frame, starting the aftermath.

Aftermath
Now begins the final standoff between the geth and quarian fleets. The shooting has stopped, but not the yelling. Legion and Tali square off. Legion wants to upload the Reaper code to the geth collective, as it enhances their processing functionality to the point of achieving true intelligence. Tali is naturally fearful of what this might mean for her people, since the quarians have already resumed attacking the geth. Shepard must moderate the decision.

Your first choice is to allow the upload, or to let the geth die. By making either decision, either Tali or Legion will protest and a second decision branch is opened: allow the upload again, stop Legion, and a Reputation check (Charm: "Rally the fleet" / Threaten: "Warn the Fleet").
 * If you allow the upload, Legion will sacrifice itself to distribute the new programming and the upload will be finalized. The Migrant Fleet will be wiped out by the newly-enhanced geth, resulting in the extinction of the quarian race. Tali will join her people, casting herself off a cliff; there is a Paragon interrupt, but even with it you will not be able to save her. Shepard will gain the geth as a War Asset, but Tali will no longer be selectable as a squadmate. If Tali did not survive the Suicide Mission in Mass Effect 2, Admiral Shala'Raan vas Tonbay will be present, and will shoot herself.
 * If you choose to stop Legion, Legion/Geth VI will attack, forcing Shepard and Tali to attack it. Several Renegade interrupts will follow allowing Shepard to shoot and eventually kill Legion/Geth VI. The quarian bombardment annihilates the geth in its entirety, resulting in the extinction of the geth collective. Thereafter, the Migrant Fleet joins as a War Asset.
 * Passing the Reputation check is the most difficult, though also the most rewarding, as it leads to the quarians and geth co-operating. The quarians will back down, and both fleets will join as War Assets. While Legion is forced to dissolve its 1183 runtimes to disseminate true intelligence throughout the geth collective, it achieves true sentience and individuality first, and is able to accept Tali's friendship before passing.

If resolved with co-existence, talking to Tali afterward onboard the Normandy will reveal that the geth are providing invaluable assistance in helping the quarians resettling onto Rannoch. They set up housing and settlements in a far more efficient manner, and volunteers are having their suits loaded with geth programmes that function in mimicking mild virus attacks, which in time will help to boost the quarians' natural immune systems so they can live normally without the environment suit. Tali admits that she can't see the quarians actually ditching the suit altogether, however, as it has become part of their culture. If Shepard and Tali are in a romance, they flirt briefly, with Shepard teasing Tali about allowing geth inside her suit and Tali assuring Shepard that the only one getting into her suit will be him.

Whatever the outcome, the mission Priority: The Citadel III becomes available.

Enemies

 * Combat Drone
 * Geth Hunter
 * Geth Prime
 * Geth Pyro
 * Geth Rocket Trooper
 * Geth Trooper
 * Geth Turret
 * Reaper Destroyer