Priority: Geth Dreadnought

The quarians have tried to retake their homeworld from the geth. Unfortunately, the geth have been upgraded by the Reapers, and the quarian fleet is trapped. Travel to the quarian home system and disable the geth dreadnought broadcasting the Reaper control signal.

Acquisition
After the completion of Priority: Perseus Veil, this mission will be added to your journal.

Squad
Tali'Zorah vas Normandy is a mandatory squadmate for this mission, unless she died in the suicide mission from Mass Effect 2 which in that case Admiral Xen will guide you through the mission but not be a squadmate. Tali has Energy Drain, which might just save your life on this mission, and Sabotage, which temporarily charms any cybernetic platform into fighting on your side. This mission is aboard a geth ship, it will be swarming with cybernetic platforms. Spam Sabotage and you'll basically have a four-person squad all mission long.

If you have it as a bonus power, equip Energy Drain on yourself.

The only remaining question is who to bring as a third teammate. Biotics are always useful--there are several choke points where Singularity is worth its weight in gold--but you'll get special dialogue if you bring James or, particularly, Ashley/Kaidan, who of course spent a whole lot of time fighting geth with Shepard three years prior; similarly, bringing Garrus along also yields extra dialogue which can vary depending on romance choices. Since EDI is an AI, it's logical to assume she'd have a lot of interesting comments to share as well. In the end, the mission itself isn't terribly hard, and you should honestly feel free to bring anyone you want, for whatever reason you want.

Walkthrough
Make your way through the ruins of the docking tube alone until you reach the end where the tube will detach, leaving you separated from your squad and the Normandy. Enter the Geth Dreadnaught. Proceed down and to the left to grab a Shotgun Smart Choke, then climb back up the ledge and circle to the right to get to a ladder directly across from where you came in. Once you have climbed up the ladder, go right and through the door. In this new airlock there's 1875 credits ("Geth Data") to the left, and your goal up another ladder on the right. Activate the controls, and a cutscene will play. Pick up the Arc Pistol before continuing; it's next to the docking controls where Tali left it during the miniature cutscene.

You will enter the geth defense network. Move along the linear path, grabbing GARDIAN Defense Data and a Sniper Rifle Enhanced Scope, and activate the bridge controls. Eliminate all geth hostiles and move forward and down a ladder. You will find more geth troops at the bottom. Once they have been eliminated, move along the path and through the door, but be careful advancing: there are Trip Mines on the floor. They can be shot to cause premature detonation, but they blend in pretty well. Grab more Geth Data (+2500 credits) before passing through the next door, which leads to a room with the Archon Visor in it, plus a couple of terminals that will update you on the progress of the battle; the room beyond it contains the Geth Pulse Rifle. Finally, you will arrive at the Operations Center where Tali intends to shut down the Reaper signal.

Of course, the Operations Center is manned, particularly by Geth Hunters, who use optic camouflage to sneak up on you and decimate you with their shotguns. Their weak spot is the fact that their cloaking device doubles as their shields, and is nowhere near perfect: if you catch the visual distortion, shoot it to make it targetable, particularly by Overload or Tali's Energy Drain. That will end the Hunter's pretentions at stealth and at having barriers right quick.

Once everything's dead, move out, being wary of trip-mines. Grab the SMG Scope and Geth Data (1250 credits) before activating the target console. Tali will inform you that you need to get to the drive core to accomplish your mission. Tali will then open the maintenance tubes and geth will flood inside, including up to four Hunters. Eliminate the hostiles and proceed to the back left of the operations center to enter the maintenance shaft. Bypass the door, then go around left to go down the ladder. Continue around and drop down a second ladder. Pick up the Geth Data (1250 credits) on the left, and go down another ladder. Finally, pick up the med kit and head through the pair of doors into the Main Battery.

This space is where the dreadnought's titanic main gun resides. Every time it fires, an electromagnetic shockwave will come screaming down and disable your shields unless you're in cover. Get used to the timing; you're going to spend a lot of the remaining mission in this corridor. Fortunately, the shockwaves are pretty darn obvious, and furthermore "in cover" just means "using a chest-high wall," not, "actually ducking." If you're popped out from cover to shoot, the shockwave will not hit you. (Presumably whatever hunk of metal you're next to is acting like a lightning rod.) This is also why Energy Drain is the absolute best bonus power to carry around in this mission: not only does it help you deal with the Hunters and Geth Rocket Troopers, it can provide a quick shield boost if you accidentally snag a Screaming Electromagnetic Shockwave.

Two waves of geth will appear the moment you enter the corridor. It's possible to just stay put and let both waves come to you; if you'd like to be more aggressive, a raised control post to the right offers flanking potential. In any case, your squadmates will tell you it's safe to move, at which point you should head up the hall and take a left to find a maintenance lock. A small ramp to the left, just before the maintenance lock, has some geth data you can salvage for 1250 credits. Once you've activated it, you'll want to sprint--unless, Keelah forbid, you have some absurd desire to find out firsthand what it's like to be fired out of the galaxy's biggest mass accelerator.

You are now on the opposite side of the main battery, but the screaming electromagnetic shockwaves haven't finished yet. You'll almost immediately be engaged by the geth. If you go straight forward from where you start, you can take a ramp to a catwalk where the geth are shooting down at you from, but this exposes you to a lot of fire very quickly. Likewise you can turn look right from your starting point and take a ladder up to the other side of same catwalk, but the same problem occurs. Do as your conscience dictates. You'll have to climb up eventually, though, because the way forward is a ladder on the far side of that catwalk. As you advance from there, you'll hit a dead end. Take the ramp on your left up to another control post, which has a pair of doors facing the main cannon. Say goodbye to your friends and the screaming electromagnetic shockwave as you go. Before entering the platform, go around the outside to the right for more geth data (1875 credits). Move onto the platform and access the platform controls. A miniature cutscene will be triggered as you and your squad progress upward, showing two geth with ML-77 Missile Launchers. They will shoot the bottom of the platform, causing it to collapse. Sprint forward toward Tali before it collapses completely. Proceed up the ladder, head past a medkit and through the door to enter the drive core. A cutscene will be triggered. The drive core will reveal the hardware the Reapers are using to distribute their control signal: Legion (or a "Geth VI" if Legion is dead), who will shut down the signal if you release him.

Grab the Assault Rifle Precision Scope and geth salvage (2500 credits) and then take either the platform or the ladder up to the top. If you took the ladder move around the outside to the data core override. Access the panel to free Legion or the Geth VI, who will then shut down the dreadnought's drive core. Multiple enemies will filter in shortly afterward, including Geth Primes. These aren't all that much more dangerous than Hunters (they certainly source less damage), but they have a ton of hit points.

Eliminate all hostiles and proceed back around the half-circle balcony to get to the ladder. From here, you're basically in the clear. You can take your time to look around, the ship will not explode with you in it. Move through the linear path until you reach the hangar. The walkway will collapse beneath you. Run up the ramp to your geth friend to end the mission.

Enemies

 * Geth Hunter
 * Geth Prime
 * Geth Rocket Trooper
 * Geth Trooper
 * Geth Turret

Trivia

 * When approaching the geth dreadnought, Joker states that with the stealth systems engaged, the only way the geth will detect them is if the Normandy's crew starts singing the Russian national anthem. This is a reference to The Hunt for Red October, where an astute American sonar operator briefly detects the Soviet submarine Red October, which is equipped with an advanced stealth system, due to the singing of the Soviet national anthem aboard the Soviet sub.
 * After the battle, talk to Cortez in the shuttle bay, he'd remark on the geth fighter craft that Shepard's squad flew out of the dreadnought, and that the craft have already been sent to Admiral Hackett's side for close study.
 * If EDI is brought along for the mission, even though her physical body is standing right next to Shepard, she would still contact them through the Normandy's speakers.