Vermire: Saren's Base

As a note here: after completing the fourth of the four main missions, you will automatically be returned back to the Citadel and after the next time you leave you'll be prevented from heading back there. If you want to complete any side quests, be sure to do so before heading to Vermire; otherwise some of them will wind up uncompletable.

Go To Vermire

A salarian spy team has some kind of info on Saren. You need to rendezvous with them and discover what precisely it is they know. After landing, you'll have a lengthy Mako section that will see you attempting to disable the planet's AA towers. Keep in mind that some of the overhanging structures that the Geth inhabit here can be entered if you get out of the Mako and look for stairwells.

You can get out of the Mako and enter these structures, if you wish.

After shutting down the anti-air turrets (check the control rooms in the facilities you come across) and opening the gates that impede your progress, you'll reach the Salarian camp, where you'll learn that Saren has cured the Krogan genophage and is planning to unleash an army of Krogans to match his geth.

Survival Of The Fittest

Speak to Wrex on the beach here, but save your game first. This is an important conversation, and you and Wrex will eventually wind up gun to gun. There's apparently only three outcomes here: either you shoot Wrex, Ashley shoots Wrex, or you use your Charm or Intimidate skill to talk Wrex down and you all walk away. Hope you've been investing in your speechcraft!

Two men enter! One man leaves!

With that done, speak to Kirrahe. He'll offer you a chance to sneak around to the back of Saren's facility and infiltrate quietly, but it will mean sending either Ashley or Kaiden to an almost certain death. You have to choose one of them to accompany Kirrahe's men; there's apparently no way around it. Keep in mind that sending your loved one may wind up preventing you from completing your romantic interactions with them. Regardless, you have to choose someone, so pick one or the other and volunteer them. For the record, we picked Kaiden, but you'll have to make another choice in a little while.

Before starting the mission, sell any unnecessary goods to Commander Rentola and hack the station nearby him to get some amps. When that's done, speak to Kirrahe to get his little Shatner-esque speech, then get ready to move on to the mission.

Assault

Your team is tasked with taking out the AA guns above Saren's base, but you also need to help Kirrahe and his men by disabling as much of the Geth defenses as possible. Begin by heading north to the communications tower near where you begin and using the machine there to disrupt the Geth communications. There's a satellite uplink to the west; destroy the targetable tower there to continue the disruption. Another target is the northern refueling bay: kill the drones there, as well as the fuel tanks.

When you reach the entranceways to the lab, check your map; you'll note that there are entrances to the north and to the south. Walk onto the beach through the gap in the guardrail and take the southern entrance; it'll require you to hack into the door controls. When you do, though, you'll soon come across a security station that will let you disable the alarms in the base. You can choose to reroute the alarms so that more of them occur near Kirrahe's men, but that'd be pretty mean.

You can disable some of the alarms to take the pressure off of Kirrahe.

Into The Labs

You start making your way up through the facility here, starting with Cell Block B. Letting the salarian here out of his cell will force you to fight him, but it will earn you paragon points.

From this point, you should try to explore the base a bit before moving on with your objectives. The Maintenance and Warehouse areas aren't important to explore, but they have enemies that will net you more cash. (Following the path from them will lead you back to the rear entrance of the base.) If you take the stairs down to the Detention Cells from the mid-level Security Office, you can find more Salarians to deal with.

When you're ready to move on, though, take the elevator up to the Lab. You'll have to kill Doctor Droyas. Put him down, then head to the security office to meet up with Rana Thanoptis, the administrator of the facility. After she unlocks the path to the communications tower, head up through the elevator for another vision…and an interesting conversation.

Disable AA Guns

Head back down to the security office; the path leading to the breeding facility is now open. Head around the outside of the Base Main Level, poking around near the stairs for obscured crates, until you reach the security control that'll shut down the first AA gun. Activate it, then defeat the counterattack and head up the elevator. Make your way through the breeding trench here to meet up with Ashley.

Choices, choices.

Ashley, Kaiden, Kaiden, Ashley

Save your game before heading up the elevator here, if possible; another tough decision awaits. You'll have to choose between proceeding forward and nabbing Kaiden or falling back to protect Ashley. Whomever you choose will wind up being the individual that lives through this mission; the other one will be staying behind. If you choose to head back to Ashley, the fight will be slightly more difficult, as you'll face off against your first Geth Prime. This hulking beast will have extremely quick shield regeneration, but you will be able to take it down if you hit it with everything you've got.

Your first encounter with Saren will occur here, no matter which team member you've saved. He's tough, but not impossible to beat, especially since you'll be taking him on three-on-one. Hit him with Overload, Sabotage, and everything else you can think of, then use your weapon skills to finish him off. He won't die, of course, but he will retreat, allowing you to escape Vermire before the nuke detonates.

Back To Normandy

When you return to Normandy, you'll have your path to the Conduit. If you completed enough of the side objectives to help out Kirrahe's men, he and they will be in your Engineering section, and Rentola will sell you some unique objects, including a couple of rare licenses. Be sure to nab them before heading to your next destination, as they'll leave the ship when you do so.