Finding the Cure

The emotional heart of Sam's storyline, and entirely optional - which makes it the one to not skip. Follow Sam's trail through Phi Robotics and the Glacial Basin to find her hidden cache with the antidote, take it to the Frozen Leviathan in the Glacial Bay, load it into the injection robot by the creature's head, and press Begin Injection. As the Kharaa pustules fade, the achievement pops and the story lands its best moment.