Description
Once you commit to the Reaper IFF mission, there's no going back and you're on the clock. Once the crew is abducted and you do one side quest half of the crew is gone, but if you take too long the crew that was abducted is dead during the suicide mission.
Do all the important stuff before proceeding. Main quests prior to this, loyalty missions, side quests, and your preferred teammate to romance.
Also this is the only time you play as someone else besides Shepard.