Recruit Professor Mordin Solus
Gain Access to the Quarantine Zone
Return to Omega. We know that Solus is in the quarantine zone, attempting to find a cure for the mysterious plague. To get there, you need to take a shuttle – it is marked on your map as 'Shuttle to Quarantine Zone'. In picking your team you should only recruit Humans, as they are immune to the virus. So that is Jacob, Miranda or Zaeed, but not Garrus (though it won't actually have an effect on the likes of Garrus, he may not appreciate being put at risk unnecessarily). If you forget you will have another chance when boarding the shuttle. Turn right at the 'Mad Prophet' to find the correct corridor to the shuttle.
You will have to charm your way in, as the zone is completely locked down. Speak to the District Guard on the door (note that the District Guards are far more accommodating than the mercenaries!).
Steer the conversation to allow access to the Quarantine Zone.
You can use your Paragon or Renegade experience to help your cause here. Take note of the warning about Blue Suns mercenaries inside, and the cut scene will end.
Locate Mordin Solus
The Blue Suns will cause you the most problems down here, but it should only be friendly District Guards in this first section (unless you provoked the doorman upstairs!).
Collect the Refined Element Zero behind the stack of crates and the ammo next to the Batarian corpse. Continue through the door and you will come to a barricade. Those manning it should let you past if the doorman radioed on ahead for you. Make use of the Medical Station in the next room and you will come out behind the barricade. Proceed through the door as you are warned about Blue Suns and Vorcha up ahead.
Downstairs you will see two figures in the distance. They are hostile, but will only attack when you reach the bottom. They shouldn't give you any trouble. It's clear that the plague is well out of control and those in these slums are being holed up by the mercenaries in the area. Hack the Bank Terminal near where the pair of mercenaries were standing. Head through the door next to it and collect the Power Cells. Hack the other door in here to find a dead plague victim. Play the logs on the computer for some XP.
Back in the main room with the plague-fire you will find an infected Batarian.
Opt to help or leave the infected Batarian
This is an opportunity to score some mean Paragon points. Firstly, however, get any information from him that you see fit. He will tell you about Mordin's unusual methods, skill in combat, and the lethality of the Vorchas roaming the area. They are immune to the plague like the Humans, suggesting that one of these two species is responsible.
In the next area, hack the door to the right to find a room with a Wall Safe and some Data Logs. Piecing together the mystery of the plague is an interesting diversion. The next room is a large one. In the courtyard area you will be accosted by Blue Suns Mercenaries. Take out the half-dozen or so that enter the room, then head upstairs. Pick up the Med Kit and salvage the Circuit Boards.
Back downstairs, work your way through the courtyard area – turning right before the barricade. Hack the locked door to find an apartment with a Med Kit and two Humans in! They are scared about the mercenaries prowling outside, so had locked the door. After stealing from their Wall Safe, you can use your Paragon or Renegade skills to convince the pair to visit Mordin, once you clear the route ahead. Of course, you could just leave them to die. It's your call.
Back in the main room continue past the barricade, making sure to take the Assault Rifle Damage Upgrade from the fallen mercenary. Head through the door and go straight ahead – you'll come across a Blue Suns versus Blood Pack fight. You can let it play out, but the Blue Suns are outnumbered.
Use the distraction to take out as many mercs as possible
You will find tonnes of Blood Pack soldiers in here, including Varren. Watch out in particular for the Pyro units. If you get caught in their flamethrowers it's nearly a guaranteed death. You will have to try and get a team mate to Pull the Pyros away, or try to regain control of Shepard and pump them full of lead. Avoid them if possible, however. Use Incendiary Ammo or a Sniper Rifle to engage from distance. They also have Blood Pack Warriors, so don't let them get too close.
With the area clear, locate the side door to the left of the barricade complete with Refined Element Zero. Go into the adjacent room and you'll find a pair of looters. You can ask some questions and deal with them aggressively if you don't agree with their actions.
In the mercenary room we were in previously, go past the barricade and through the door. You should see a sign for the clinic. Remember to collect all the ammo in the room before heading out.
Head for the Clinic
You should be out of harm's way now – Mordin has got the Clinic well guarded by friendly troops. Inside you can change your weapons using the Armoury, and talk to the various patients milling around. Most of them hate Humans. Move through the Clinic until you get to a crossroads of rooms, the forward path blocked by a barricade and some guards. Go left first, collecting the Platinum, Med Kit, Palladium and a Medi-gel Capacity Upgrade from the computer. Now go into the right-hand room to see Mordin.
You have Renegade and Paragon opportunities in this conversation. Ask him some questions, most importantly to recruit him into your squad. He'll tell you that he first needs to spread the cure for the plague that he has developed across the entire slums. The sensible way to do this is through the ventilation system. Mordin gives you the cure (as well as a new weapon) and sends you on your way. He also requests that you look out for his assistant, Daniel. Be sure to mention the Batarian plague victim at the entrance to the slums too.
You can also take Power Cells, Refined Element Zero and Refined Iridium from this room. Head out of the Clinic using the other door (i.e. not the one you came in through). It is located next to the grumbling Batarian Patient.
Up the stairs, the first room on the left is packed with Blood Pack soldiers, including Pyros and Warriors. Deal with the fleshy ones using Incendiary ammo, while your Grenade Launcher is great against the Warriors.
Head for the Clinic
As you move up, more enemies will appear. Remember to target explosive containers to deal the most damage. Clear the room and use the Medical Station. You will also find a hackable Gambling Terminal and upstairs a Bank Terminal. Continue up the next set of stairs too and you'll find a door to the left. Inside is Mordin's assistant, Daniel.
He was delivering a cure, but as Humans are suspected of creating the virus, and Batarians are naturally suspicious, the situation has escalated. Either talk the Batarians down or agree to fight them, receiving Paragon or Renagade points as a result. Daniel will return to the Clinic with the hostilities defused (or eliminated).
Activate Environmental Control
Go back into the previous corridor and into the door straight ahead. More enemies must be dealt with inside. Proceed downstairs with the area clear and you'll come into a tricky room. Take cover behind the door and snipe any RPG soldiers you can see on the balcony in the distance. You should try and eliminate these enemies before heading downstairs, as their rockets will easily target you with a height advantage.
TIP: Use the explosive canisters to take out the upper enemies.
With the upper ranks defeated, head downstairs and mop up the rest of the room. As always, watch out for Blood Pack Warriors. Collect all the ammo in the area and proceed out of the door at the back of the room. Some Blood Pack troops are in the next corridor, and you should quickly eliminate them. In the right hand room you can salvage some Spare Parts and get a Med Kit, whereas the room to the left leads upstairs to Environmental Control. Inside you'll find quite a welcoming party.
An ugly reception greets you.
Kill the Vorchas and Blood Pack troops, focusing on the RPG units and the Pyros. Head to the large array of computer terminals on the far wall and interact with the orange controls. You have now inserted the cure into the system. All that's left to do now is turn on the fans.
Enemies will pop out as soon as you turn around. Take cover and clear them out, before proceeding down the stairs to the right (near where you came in). More Blood Pack emerge from the door down here.
Ensure the Pyros don't get too close, and move up. Interact with the orange terminal as before to get the first fan whirring. Another ambush awaits as you leave this room. In particular you should watch for Pyros as you round the corners and move up the stairs. Try to wait until they are in range and ensure the area is clear before continuing.
TIP: Send your team up the stairs first to ensure the Pyro threat has diminished.
Back in the middle, enemy soldiers with rockets will attempt to pin you down from the balcony on the other side. Use the pillars to prevent them from getting clear shots on you in numbers, and equip a sniper rifle to pick them off.
Cover behind the pillars.
As long as you are behind cover, the rockets will not cause serious damage. Watch out for ground troops (Pyros again) as the Vorcha above are defeated, and press on into the second fan room. Interact with the terminal and you'll be helpfully warped back to Mordin. Mission complete!
Party Member Acquired: Mordin Solus
Insert the cure into Environmental Control and activate both sets of fans.
Accept the gratitude of all involved and you can choose to either return to the ship or explore the Clinic some more. There's not much left to see, though the Human couple you may have advised to follow your path of destruction have arrived, as has the Batarian you may have asked Mordin to help. Daniel (Mordin's assistant) also needs someone to talk to about Mordin, but it's inconsequential. When ready, press B (Xbox 360) to return to the Normandy.
Speak to Mordin to explain what he's here for, catch up with the crew and check your personal messages. You'll find one from the wife of a man on Garrus' fateful assault team and a message from Chakwas regarding possible face regeneration. Her tip is to be nice, and your skin will heal.
When ready, return to Omega via the Galaxy Map so we can finish our business there. There are just a few side-quests to tie up. You could, of course, head to the next objective if you are going for a quick play through, in which case skip ahead.