Story-related and cannot be missed. This mission is acquired after the completion of two of the first three plot worlds: Therum, Noveria, or Feros.
Once available, just travel to Virmire and follow the quest objectives.
*** Spoiler - click to reveal ***This are the steps needed to finish the mission:
- Drive Mako until you reach gatehouse #1, clear all active Geth that are nearby, open the gate
- Drive to the gatehouse #2 and kill Geth Collosus and three Geth Armatures
- Clear gatehouse #3 and drive to salarian camp
- Have a discussion with Captain Kirrahe
- Solve Wrex's problem (I'm trying to be as vague as possible if that's your first playthrough)
- Talk with Kirrahe once again, pick one of the squad members to assist him, and start the assault
- Use either the main route towards the base OR secure three optional objectives - this is recommended for Kirrahe's team to survive (disable comms, destroy the antenna, destroy fuel tanks)
- There are two routes to get into the base - pick one you prefer (one will allow you to focus enemies interest on your squad)
- Go through each location one by one until you reach a room with Asari, Rana Thanoptis. Decide what's her fate and ask the questions you like
- Go to the next room and have THE conversation. Yes, this is literally one of the best moments of the game
- When you have a short chat with Joker, you'll have to defeat some krogans and get to your team
- The rest of the mission is pretty straightforward and I'll leave the decisions you make without spoilering to much