They're all immune to HLV...until they aren't.
When a mysterious illness breaks out at the armory, Grace and the others scramble to save everyone when it becomes obvious HLV has mutated again. Jason, a recent arrival, knows of a research facility that might hold a cure, or at least a vaccine, so she and Clint go with him to see if they can find a way to save their people. Rafe joins their group after he steps in to save them...