Once you’ve completed any and all side-missions you had, it’s time to head back to the Citadel. Joker tries to dock with the Citadel, but no response is coming back to give the Normandy clearance to land. He finds a distress signal on another frequency, and you learn Cerberus forces have overrun the Citadel. Shepard knows of a landing zone near C-Sec headquarters, and takes a small squad to infiltrate the Citadel.
As the shuttle pulls into the landing zone, you see C-Sec embroiled in a battle against Cerberus forces. Make your way through the battle, taking down any and all foes you see. After you’ve eliminated all the Cerberus soldiers, head towards the large door at the top of the steps. An injured Commander Bailey is on the ground nearby, and you need to talk to him to get the door open.
Bailey searches the computer in the next room for information, and finds that the Salarian councilor is in danger. Bailey believes she knows who is behind the attack on the Citadel, and it’s up to Shepard to rescue her.
Head through the doors behind the computer terminal, and enter the next room. There are some Assault Troopers at the far end near another door. Engaging them draws some Guardians into the room. Take every last one of them down. Move up the steps on the left, and you’ll encounter more Cerberus forces including Engineers and Assault Troopers. Kill them, and then take a moment to check the area to the right for an Assault Rifle Piercing Mod II and Med Kit. To the left of the Mod, you’ll find a C-Sec Corpse to examine.
Bypass the locked door, and proceed into the next area. More Cerberus forces await, though they are unaware of your presence until you start firing. Eliminate all of them. On the left hand side of the room from where you entered, you’ll find a Recon Hood on a table. There’s a bathroom on the left as well, and inside you’ll find an M-76 Revenant. Head back out to the main room. Near the entry way to the next hall there’s a Medical Station. Move through the hall, and hop through the broken window on your left. Inside there’s more ammo and a Sniper Rifle Extended Barrel III. Along the same wall, you’ll find a locked security box. Shoot off the latch, and then activate the Elevator Door. Jump back through the window behind you, and enter the elevator.
The next hall is filled with dead bodies. Proceed to the end of the hall until you reach an open doorway on the right. Inside you’ll find a Pistol Piercing Mod III and a computer terminal to examine. Head back out, and continue down the hall to the next open door. Move inside, and activate the Door Control on the first computer terminal. Turn back out, and head back towards the elevator door you came out of. To your right, you’ll see a newly opened door. In the room there is an M-358 Talon under the shelves, and a Datapad.
Exit the room, and continue down the left hand side. There’s another door on the left. Enter that room to discover the Securitel Helmet and Pistol High Caliber Barrel III. Leave this room, and move back into the room across the hall. In the middle of the hall, there’s a dead C-Sec solider. Next to his body you’ll find an M-25 Hornet. Proceed into the next room, and through the next set of doors. Some Assault Troopers are standing guard. Take them down, and then move through into the next open area.
Hop through the first broken window on your right. There’s a Datapad on the counter. Grab it, and then move to the broken windows at the other end of the kitchen area. Cerberus troops will drop in. After the firefight, head all the way down the hall, and turn right just before the next doorway. In the corner you’ll find a Sniper Rifle Piercing Mod. Head back through the doorway, and up the stairs to the Executor’s office.
Once there, you find that everyone is dead. The Councilor comes out of hiding from the room below, and is confronted by an unknown assassin. She reveals that Udina is behind the attack, and is then cut down by the assassin. He escapes, leaving Shepard and his squad to try and hunt down Udina and the other Councilors before it’s too late.
As Shepard tries to head off Udina in a shuttle, the assassin reappears to disable the engine. The ship crashes in the Presidium, and now you’ll have to make it the rest of the way on foot. After exiting the ship, you’ll be confronted by some Cerberus forces. Proceed with caution through the first section, and then hop over the gap into the next area of shops. Here you’ll encounter some Nemesis soldiers, and a new foe, the Phantom. Take every last Cerberus troop out, and then look for the broken guardrail on the right. Drop down to the next level, and head through the service tunnel in front of you. When you get back outside, look to the right, and drop onto the ledge. Jump across the gap straight ahead, and climb back up on your left. There are some Credits to salvage immediately to your left. Turn around, and head through the opening directly in front of you.
After you jump down from the open window, an Atlas mech drops in. Take out the Atlas and the few Assault Troopers assisting it, and proceed up the steps on the left. Once you open the next door, you’ll witness the assassin and some Phantoms entering the elevator. They take off before you can stop them, but there’s another elevator on the left you can use to chase them down. Bailey will slow the assassin’s elevator until you’re able to catch up with it.
Shepard and the squad hop onto the roof of the elevator car, and give chase to the assassin’s elevator. As you approach the other car, you have to shoot out the Power Conduits. That will stall the assassin momentarily. As you continue your ascent, Cerberus forces will try to jump onto your elevator. Take them out as you continue upwards. After a few waves, you’ll get the chance to jump onto the Council Elevator. Make the jump, and a lengthy cutscene will begin.
The Council exits the elevator, and tries to rush to the shuttle. Cerberus has disabled it, but before the Council can retreat back to the elevator Shepard cuts them off. Either Kaidan or Ashley will be guarding the Council. You’ll have to explain your motivations, and either reason with Kaidan/Ashley, or take them out. Udina will try to escape again, and you have the option of shooting him, or letting Kaiden/Ashley take care of him.
After taking down Udina, Bailey arrives to inform you the Cerberus forces have vacated, and the Citadel appears to be safe for the time being. More than a dozen new side quests will open up on the Citadel, so before returning to the Normandy it’s best to take the time to explore. Once you’re back on the ship, you learn that the Quarian’s require your assistance.