When a deadly mutation makes 2/3 of planet into inhuman creatures, the first generation thereafter seem to be immune to the mutation and the government was perfectly content in kill of the inhumans until the new generation caught the mutation. But instead of changing them into inhumans it did something different to their DNA. Cassie McCall part of the new generation who hasn't caught the disease yet works for the government and is an assassin for one of the teams who kills the inhumans. When she's told a member of her team has the disease and is suppose to kill him off she can't bring herself to do it and ends up helping him escape.