Bah! It had to happen... zombies getting loose on a world where there's superheroes.
The core concept of the 'Your World No Longer' line is that some power has taken over the world and it's up to humankind to do something about it. Superheroes apparently fall into the bracket of 'humankind' for these purposes although at times this seems to be a case of "For it's Tommy this, an' Tommy that, an' "Chuck 'im out, the brute!, But it's "Saviour of 'is country" when the guns begin to shoot" (with thanks to Kipling) if superheroes are viewed with suspicion by the authorities of your campaign world. No matter the 'power' those who want to see them gone and to reclaim their world are known as the Resistance, and a framework for running adventures with this concept are provided.
Anyway, in this case, it's an infestation of zombies that needs to be dealt with. As this is the first 'Your World No Longer' product, there's a lot of general detail about how to organise the Resistance and how to involve the party. Or even if to involve the party, they might be working independently of non-superpowered resistance groups, or they even might BE the Resistance if everyone else is too cowed to act. Organisation, recruiting, tactics, resources - such useful topics are discussed in turn and are of general application, whatever threat has to be dealt with. There are even thoughts on how to introduce this kind of thing into an existing campaign, as opposed to starting out with a resistance-based game from the outset.
That dealt with, the discussion moves on to the specifics of creating a zombie threat for the superheroes to deal with. Some new rules are introduced, mostly attack variants, and new flaws; and then comes details of how to model zombies using the Mutants and Masterminds ruleset. Various options are given, including of course the shambling brains-obsessed movie zombie but there are other options including infected animals. These are presented as templates to apply to normal people and animals. However the plague of zombies arose - that's left for the GM to determine - all are regarded as being infectious and capable of passing on their zombie state to others, normally by biting them or other exchange of bodily fluids. And what happens if supervillains get 'zombiefied'? Some, if you use the 'intelligent zombie' template, will still have access to their superpowers.
Next comes material regarding the Resistance, including what likely tactics will be employed against zombies and how to involve the party. Then comes advice on actually running the game including creating the right atmosphere, one in which the characters feel besieged and under threat however well-organised and resourced they are. This is followed by some scenario ideas including the initial outbreak (on Manhattan Island of all places - would they even notice zombies there?) as well as rescuing people and even finding a cure, and finally how to deal with the aftermath of the zombie plague once defeated.
This is an outline, a bunch of concepts that you will need to consider if introducing these elements into your game. They are good - but you still have some work to do before you will be ready to play.