Priority: Earth

Allied forces are assaulting the Reapers above Earth. Hammer must get a team of soldiers onto the Citadel in order to activate the Crucible. Coordinate with Anderson to get to the Citadel beam.

Acquisition
This mission is acquired immediately after Priority: Cerberus Headquarters.

The dialogue options to start or postpone the mission do not affect Renegade or Paragon scores.

As noted previously, this mission is part of the end-game sequence. By beginning Priority: Cerberus Headquarters, the player forfeits the freedom of the galaxy and can (from that point forward) not fly anywhere except Earth itself.

Preparation
No side missions available at this point, every system except Sol is unavailable on the galaxy map and no more war assets can be obtained. The effective military strength (EMS) modifier can still be increased by completing Mass Effect 3: Galaxy at War activities. Having more EMS will result in Earth and your forces not taking as much damage over the course of the counter-attack.

In choosing your powers and equipment, consider prioritizing either a sniper rifle or some sort of spammable direct-damage ability. Remap your powers if necessary. During this mission, if you're not in a cutscene, there will be bullets flying at you, often from many directions, so having being able to pop out of cover, deal damage and duck again will be your best friend. The more standard FPS run-and-gun build will require as much shield regeneration and armor as possible. In that case, consider replacing your extant Bonus Power with Tali's Energy Drain or EDI's Defense Matrix: they refill your shields when used (EDI's will need to be "double-tapped", but you can do so almost instantly), thus providing a useful "panic button."

In choosing squadmates, make absolutely sure that someone provides fast, accurate damage: your every shot must count, because you won't get many. Beyond that, your decisions will depend at least partially on your own build. You do get to choose a new squad before the final push, so feel free to experiment in the first half of the level.
 * James has damage output, but not a lot else. Thankfully, you can retrofit him as a sniper by giving him the M-99 Saber.
 * Ashley at least comes with an actual sniper rifle.
 * Kaidan's powers let him counter barriers and armor, both of which this level is overflowing with. Consider emphasizing his recharge speed and providing the firepower yourself.
 * Liara is probably the worst choice for this mission: she lacks firepower and durability, and will find her biotics devalued by all the shields, armor and wide-open level design. But Singularity and Stasis all but guarantee a dead enemy, especially to a player with good aim.
 * Garrus has the ever-useful Overload, as well as a sniper rifle. Just make sure to switch him to assault rifle during
 * EDI, like Kaidan, can smash both shields and armor, but lacks his firepower and durability. If you bring her, make sure to max her Defense Matrix.
 * Tali has decent mezzing and damage output via her shotgun, but its short range exposes her durability issues. If you bring her, make sure to max out Energy Drain.
 * Javik's powers aren't the greatest, but he does come with decent firepower via his assault rifle. This is especially true if you've equipped him with that do-all wonder-gun, the Particle Rifle, but since it's the right choice for every character who can wield it, we're assuming you know this and concentrating on other weapons.

Take Out AA Battery
Hackett and Anderson will walk Shepard through the plan they've put together (which basically amounts to, "Ships shoot Reapers while ground forces try to reach the Conduit"), but like all battle plans, it's tossed out the airlock the moment the Normandy pops through the Relay. Instead of flying straight in, you'll need to stop and take out a Reaper AA Hades Cannon so that the ground forces can get through. Prepare for the fight of your life; this first one is an endless stream of Cannibals, Marauders and two Brutes. Cortez will also be shot down here, which might lead to his death if you haven't taken the time to talk him through his personal problems. After you finally clear out your LZ, you'll get to advance to what seems to be the remains of a house (if the stove is any indication), where you fight off more hostiles and then get to use a M-920 Cain to knock down the AA gun. There is a medikit close to the Cain.

There should be a second Cain nearby. Grab it, and then retreat either back to the west side of the house or to the south side of its eastern half: you need room to use it on the oncoming horde, which includes a Banshee (which may be named Morinth). Just be aware that you can miss, at which point you'll be nose-to-nose with an angry (and presumably radioactive) Ardat-Yakshi husk.

When extraction arrives, don't try to fight: just run for the shuttle.

Meet with Anderson
You're now in the Forward Operating Base, where Hammer will assemble before making a move on the Conduit. Spread throughout the base you will encounter and have the opportunity to meet however many of the various squad-mates you may have had. The number of squad mates can range from as few as three (if Garrus did not survive Mass Effect 2, if Tali did not live through the suicide mission or Priority: Rannoch, and if the Virmire Survivor died during Priority: The Citadel II or was not allowed back on the Normandy during its aftermath) to as many as seven (if the From Ashes DLC is installed and Javik was recruited). You will also run into the current leader of the krogan (Urdnot Wrex / Urdnot Wreav), Primarch Victus, and, via a vidcomm terminal in one of the buildings, Cortez and Mass Effect 2 squadmates (if they are alive). Enjoy these last moments; these are basically your final farewells to your favorite characters.

Note also that there are a number of one-way doors leading up to Anderson, so don't panic if you haven't found certain characters. Liara, Urdnot Wrex/Wreav, Javik, EDI and Tali are all on the other side of the walkway and its turret-gun minigame. There are also two medkits in the same room as Liara, a datapad just through the door to the right, and another medkit where Anderson is. After you speak to Anderson and make a speech to your squad, you'll have a last chance to pick your squad and make adjustments. Then it's time to face your destiny.

Approach the Conduit
Advance until you see the M35 Mako blow up; there you'll need to swing round its burning hulk and go to the right to reach the street. You'll see another Mako there, which will eventually be set upon by a Harvester. Ignore them; they'll eventually sort themselves out, and in the meanwhile there's a Banshee, a Ravager and other infantry gunning for you.

Afterwards, you'll receive a call from a fellow Hammer platoon that is pinned down by Reaper forces. The door leading to them is in the far left corner, and it leads to a parking garage where husks and two Brutes will set upon you. Focus on evasion and let your squadmates do the damage. Then find the ladder and climb it.

The ladder leads to an apartment where a Ravager, supported by other infantry, fires down into the street. Wipe them out to see the platoon advance, and then loot the apartment, which has a lot of ammo, and two medkits, one in the bathroom, and the other under a corpse outside the bathroom. You can also examine a computer in the apartment after the loot. Then try the door. It's got a surprise, but a Singularity will handle it. (Spoiler!!! Some Husks will jump at you.)

The street below is a great place to use a sniper rifle, if you have one. Just beware of the Ravager far down the street, and the Brute that will smash its way out of the shop on the far right corner of the intersection. Incidentally, that's your next target. Breaking in through the windows will be easy, but the next fight takes place in its back room, which is a maze of shelves and crates. It's very easy for the enemy to flank from the left. Of course, it's very easy for you to pre-empt that by flanking them first, but that'll expose you to a lot of fire. Having a biotic to send is a great idea. There is a medkit on the shelf just before the row of terminals before you re-enter the street if you need it.

Once you've made it through this business establishment, jump back out onto the street. You'll see a huge blown-out pit in the intersection with a concrete pillar slanting across it. Run for this pillar: it's the only cover around when the shooting starts. Then continue across the intersection to the business establishment on the opposite corner (evidently a restaurant of some sort). More enemies lurk there, but because you have two windows to attack through, it'll be easy to catch them in a crossfire.

Through the door is the restaurant's back dining room, which is a different question. It's got a big window looking out onto a back alley, where a lot of badguys are going to come from; an obvious solution is to dash to one of the tables and take cover there, but the restaurant's back door is parallel to the door you came through, and enemies can flank you from it if you use the tables for cover. Fortunately, there is a medigel dispenser near the window, which might come in handy when the Brute arrives. If you look under a counter close to where the medical station is, you can also find a row of thermal clips lined up nicely in the drawer should you need to replenish your ammo.

Finally you get to head out into the back alley. You'll be drawn down a long corridor which is remarkably empty. ...At least, until two more Brutes come smashing out through the wall. Retreat and take them out, and then go through the hole they opened, as it leads to a door and a cutscene. Try to relax as you watch it. You will need all the calm you can get.

Thanix Missiles
You'll find yourself on an intersection with concrete barriers strewn every which way, shops on the near corners, a Thanix Missile truck in front of you and another one about thirty meters up from you. Get used to this zone and its three entrances (the left side, the front which faces the reaper, and the right side) because the fighting for the rest of the game takes place here.

The first thing that happens is that your objective changes to "Secure Missile Battery"--so clear the first Thanix truck you come to and wait for the console on the side to become operable. You need to use them against the Reaper, which requires you to link them up with EDI up on the Normandy. The second thing that happens is that your objective changes to "Defend Missile Battery," and reinforcements arrive: first a wave of assorted ground troops out of the front; then another from behind; then a radio message from another element of Hammer company, who tells you that you're about to get hit from the left. She's not lying: Marauders and two Banshees will arrive. There is an M-560 Hydra in the shop on the left, but you might want to save it for later. Be careful: no matter where you're hiding, you're vulnerable from the other three directions.

Once the zone is clear, EDI will announce that the missiles are ready to fire. Considering that there's a second truck of them just up the road, guess whether these ones hit. They don't. The Conduit is distorting their aim beyond even EDI's ability to correct. You're going to have to bait the Reaper into coming towards you and out of the Conduits distortion field.

The most important thing to keep in mind during this next portion of the level is that you have now gotten the Reapers attention. As such, YOU MUST keep a close eye on the Reaper who will be firing its insta-kill laser at you. The laser will strafe down the street in a reliably straight line, just make sure you are on the other side of the street or bunkered down in a building as it does not seem to cut through the shops. A useful tactic is to pull up your Power Wheel every few seconds, whether or not you need to use a power, and turn your view to find the Reaper's laser. Know where it is, know where it's heading, and avoid it at all costs; It can cause you more Game Overs than every other thing in the rest of the level combined. Note that the laser will also instantly kill any enemies that walk into its path, as well. You will now be faced with a squad of enemies consisting of up to six Brutes (who seem to like coming in packs of two and can come from any of the three directions), a Harvester who lands right in the middle of the intersection, and up to two Banshees. The exact numbers seem to vary and may relate to your Effective Military Strength. Consider using the Hydra on the Brutes, but don't waste it on the Harvester, which can be dealt with by hiding in the other shop (the one that didn't have the Hydra) and using the corner to block incoming fire; in this way, the Harvester is actually the easiest thing to kill. As this is the last time you will ever need your Medi-gel, feel free to use it liberally. There are two medkits in the shop where the Hydra is and a medical station in the shop on the opposite side of the street.

Once you wipe them out, two infantry squads led by Banshees will begin spawning, in addition to the continuing Reaper laser. The music will also start up, as though suggesting that the fight may have gotten a bit intense. This flow of enemies seems to be infinite, so put more priority on surviving and less on fighting. Try to stay on the south side of the map (near the fired Thanix truck) as much as possible; the enemy will have that much more trouble shooting you. Also, if you go back to the shop the Hydra was in, the laser actually can't hit you, and it's a much more viable spot to fight from with only two bosses flapping about. Once this phase starts, all you actually have to do is stay alive until EDI gives you the all-clear and you can fire the last missiles.

Once the option to activate the missiles appears, there's no reason to fight whatsoever; ignore any enemies and just run for the panel. Soldiers, Infiltrators and Vanguards may want to use Adrenaline Rush, Tactical Cloak or Biotic Charge respectively. Once you hit that panel, you can chock up one more Reaper kill on your tally.

The Citadel
It's now time to go for the Conduit, but Hammer is badly depleted and overwhelming enemy reinforcements are on the way.

RUN.

You'll be caught by a laser explosion and pass out. Once the cutscene is over, you'll find yourself badly injured and alone. Continue on to the Conduit. You'll have to defend yourself against three Husks and, as the last killable enemy in the game, a single Marauder, but you'll be in slow motion whenever an enemy shows up, and lining up your shots shouldn't be hard. Once at the Citadel, continue making your way forward; you'll be speaking with Anderson along the way until you reach a platform where he is standing. The final conversation between you and the Illusive Man begins here.

There are several conversation branches with associated Reputation checks, but the conversation's outcome hinges solely on the final choice. In order to use the final Paragon/Renegade conversational options with the Illusive man, you must have used a Paragon/Renegade options on him at every other opportunity in the campaign, including the meetings on Mars, Thessia and the Cerberus base (which were discovered through other dialogue options.
 * If you are able to choose the Paragon option, the Illusive Man will realize that he is indoctrinated and use his sidearm to commit suicide.
 * The threaten option will offer you one Renegade interrupt; non-reputation dialogue will give you two. Not taking the final interrupt, whenever it happens, will result in a Critical Mission Failure. After you interrupt, the Illusive Man will have a few final words as Shepard opens up the Citadel and allow him to see that which he has always loved best: Earth.

Shepard will then open the arms of the Citadel, allowing the Crucible to dock. Sit back and prepare yourself to make a difficult decision.

Enemies

 * Banshee
 * Morinth
 * Brute
 * Cannibal
 * Destroyer
 * Harvester
 * Husk
 * Marauder
 * Ravager
 * Swarmer

Aftermath
The remainder of the game will depend heavily on the player's Effective Military Strength. The higher the EMS score, the 'better' the player's ending will be. The player is given the choice between Destruction, Control, or Synthesis.


 * If EMS is between 0-1749, only the Destroy option will be available and it will result in massive physical damage to the galaxy.
 * If EMS is between 1750-2349, the Control option becomes available, but both choices will cause some substantial damage to the galaxy as well.
 * If EMS is between 2350-2649, the Control option will cause no physical damage, although the Destroy option will still devastate the galaxy.
 * If EMS is 2650+ the Destroy and Control options to cause no physical harm to the galaxy.
 * If EMS is 2800+ the Synthesis ending becomes available. This is considered to be the 'best' ending.
 * If EMS is 4000+ and only if Destroy is chosen, Shepard is seen barely alive, gasping for breath. (If Anderson is shot by the Illusive Man, the player needs 5000+ EMS to see this short scene)

The Destroy (red) option will result in the destruction of all synthetic life; every Geth, Reaper, and even EDI. The Crucible will fire a beam/pulse into the mass relay network, spreading the blast across the galaxy but destroying every relay in the process. The Citadel will break into pieces.

The Control (blue) option results in Shepard sacrificing his corporal form to command the Reaper fleet. The Reapers will then leave Earth; presumably the will of the Commander. Instead of firing a beam, only a pulse will be released from the Citadel. The pulse spreads throughout the mass relays network, causing them damage, but not as much as the other endings. No harm will be caused to the Citadel, which then seals itself.

In the Synthesis (green) ending, Shepard adds their energy to the Crucible's, thus creating a new, synthesized DNA. The Catalyst explains that this is the best option, since synthesis is the pinnacle of evolution, and will render the Reapers obsolete. The Crucible emits a green light/beam, altering all denizens of the galaxy on the genetic level. The Citadel and mass relays are destroyed during the Crucible's use.

In every ending, the Crucible's blast causes the Normandy to crash land on an unknown planet. The fate of the crew will relays on EMS:
 * If EMS is substantially low (below ~2000), nobody will survive the landing.
 * If EMS is moderately high (but less than 2800) and Destroy or Control are chosen, Joker and the player's most-favored crew mate steps out of the ship.
 * If Synthesis is chosen, Joker, EDI, and the player's romance option will step out of the ship. Joker and EDI will embrace.
 * If EMS is 4000+ then three crew members will exit, regardless of the final choice.

After the credits, a scene will be shown in which an old Stargazer tells tales of "The Shepard" to a young boy.