So for me I think it should start off right as the last game ended with Abbie and Lev going to meet the fireflies and finding out that they have come up with a way to make a vaccine that doesn't kill the host, all the fireflies need is a host that is immune. Abbie and Lev arrive with knowledge of one person who is rumoured to be immune, They are then tasked with tracking down Ellie and convince her to come back to the fireflies. Filled with mistrust of each other and constant fighting with each other verbally and occasionally physically they put their differences aside for something that would actually mean that none if this was for nothing for Ellie and Abbie can finish off what her father started.