This work contains advice and additional rules for running Trail of Cthulhu in a post-apocalyptic setting (as if either the Mythos or an apocalypse weren't enough to cope with on their own). This pitches the Investigators as some of the few human beings to have survived whatever disaster occurred, trying to cope with the situation... and finding that the Mythos has survived as well! Or may even have instigated the apocalypse...
Information here ranges from designing your own apocalypse to new Occupations, Skills and Drives appropriate to a post-apocalyptic world (and notes on how existing ones will work in the new setting). Apocalypse World assumes a Pulp style of play, although it may not be quite what you are used to. It also splits the setting into two periods: Aftershock, when the apocalyptic event has just happened, and Wasteland, set some time later when things have settled down a bit, or at least folk have realised that this is their new reality.
To start with, however, you will need to decide what happened to bring about the apocalypse, to destroy the world as you know it. Certain criteria are set. Humanity is well-nigh gone, some 99.99% of human beings have died in the event although the Investigators have survived (or their ancestors did if you have taken your Wasteland plot a generation or two into the future). The effects must be global, there's no scampering off to take refuge in an unaffected part of the world (well, maybe the Antarctic bases have survived...). Oh, and Mythos entities are taking the opportunity to arise, even if it wasn't them behind the apocalypse in the first place. So, the cause of the apocalypse may be human-driven, it may be the Mythos or it could be a vast natural disaster - the next dinosaur-killer asteroid, perhaps. Or a disease, or earthquakes or... get the picture? Life on earth is quite fragile when you start thinking of ways to wipe it out.
This is a thumbnail sketch, and there's even a diagram provided... but there are also whole sections on Causes, Disasters and the Casualties of the event to help you set it all up. Depending on your chosen Cause, humanity may or may not have any hope of surviving, see any prospect for a long-term future. Whatever the mechanism you decide on, the world ended on 2 November 1936 (if it was something that happened relatively slowly, like a disease spreading, it started earlier but things came to a head then). This means that if you want a nuclear holocaust, you will have to mess with history a little as research into atomic weapons did not begin until 1939. Some notes on how to go about this are provided.
Interestingly, though, you are encouraged to go beyond the suggestions, to redraw the diagram. To set the tone of your game, you are invited to visualise four dials. Their settings will influence what your game is like. The Humanity dial looks at how survivors relate to one another. Do they work together or is everyone they encounter a threat or a resource? The Time dial tells you how long ago the apocalyptic event happened. At zero, it's... happening right now and your Investigators have a grandstand view. If it's high, the Old Days are things of memory, or even forgotten. All people know is the harsh reality of now. The Weird dial is a measure of how strange things have become (apart from the collapse of civilisation itself, that is). Are there mutants or people with psychic powers wandering around? What sort of monsters are loose upon the world? The final dial is the Adrenaline dial. This measures the balance between madcap pulp-style adventures and grim struggles for survival. Will the Investigators watch the world they knew decay around them... or will their adventures give them the opportunity to do something about it? Either can work, or something inbetween, it all depends on the style of post-apocalyptic adventure you want.
Next is a survey of Occupations. Some are existing ones - what on earth is a Socialite to do now? - and others are new to the situation. All give ideas about how an Investigator with that Occupation can use his skills to best effect. Remember that it will depend on how long ago the apocalypse happened: if it's happening now Investigators can have modern Occupations that they will have to adapt to the new situation, but if it was many years ago when the Investigators were children or not born at all, they may never have had the chance to follow certain careers.
A section on Drives follows. What makes each Investigator want to actually investigate the horror around him, rather than hunkering down and concentrating on survival? Some of the regular Drives in Trail of Cthulhu won't really work at all in this setting, but others really come into their own. There are some new ones too, like Preservation of Knowledge and Witness (who wants to record what is going on, even if he isn't sure there's a posterity to record it for). Then comes a list of Investigative and General Abilities, honed to the apocalyptic setting. There's one change from the core rules: having an Ability does not mean that you automatically have access to whatever tools or equipment you need. Finding them can be part of the adventure, after all. There are examples of how to use each Ability, and suggestions for the Keeper as well.
Next comes Sanity and Stability, beginning with a look at Sources of Stability and how they work in this setting. With most human beings dead, this may mean - especially if the apocalypse has only just happened - that your Sources of Stability have died too. Perhaps it is their memory that keeps you going. Or maybe you don't know what happened to them and the search and hope is your motivation, what you cling to. Of course, this - and Pillars of Sanity - provide targets for the Keeper. There's plenty here to help you make use of them in the game. Mental illness and defence mechanisms round out this section.
The Equipment section comes next. Some things, hitherto rare, are easy to find - or to take at will (consider a jewellry shop - now you can pilfer it to your heart's content, with no store owner to complain, no police to arrest you!). Others will have to be scavenged for, you cannot go down to the shops to get them. And you might have competition for resources. There are rules for scavenging and for making equipment here, as well. Another way of getting hold of the things you need is barter. Find someone who has that thing, and bargain with him as to what he wants for it. Perhaps something else that you have (or can acquire for him), or maybe you can do him a service. A defining characteristic of the post-apocalyptic setting is that normal activities become much harder... but don't get too bogged down, unless the focus of your campaign is on actual survival.
The next section is The Afflicted. Of those who survived the apocalypse, some are... not the same any more. They may look different or have new and strange mental powers. Needless to say, 'normal' humans treat them with suspiction if not outright hostility - and many Afflicted hide their differences as a result. The cause of these changes will depend on why you had an apocalypse in the first place... and it may be that nobody knows just why (the Keeper should, but he might not be saying). Moreover, Afflictions can be acquired - and there's an interesting way to weave these into your Investigator, by giving him Affliction Points rather than Improvement Points: they are used in the same way to improve his capabilities, only now those increased capabilities have a strange origin, an unnatural expertise that cannot be explained in a normal manner. There are other weirdnesses as well, and of course all are pretty disturbing particularly when encountered for the first time. There's a discussion of psychic abilities and what can (and cannot) be done with them. For those who choose to use Affliction Points to improve Skills, there's an interesting discussion of how the way you use that Skill will change. All quite disturbing to behold, no wonder Stability checks may be called for!
This is followed by a section of Mythos Entities, remembering that with the apocalypse (whatever its original cause) they're now able to walk the face of the earth more freely than before. Here are notes on many of them, what they are after and what the might do now that they've been unloosed! Finally in the 'open to all' part of the book (although it's possible that the Keeper may restrict quite a lot to keep the underlying mechanics secret) is a very useful section of Tips for Players, which all prospective players really ought to read. Here it reminds them that each adventure has a core question, which the Keeper should lay out. Don't stop until it has been answered! There are thoughts on using Drives to best effect, investigating horror no matter what, not getting sidetracked (particularly by mundane matters like day-to-day survival... yes, it needs doing but that's not what the game is about), and building relationships. Read it and be mindful of it during play.
Moving on, Building Mysteries is designed to aid Keepers in devising and running a strong campaign in the apocalyptic setting. Starting with the basics, it walks through deriving a fundamental question for the Investigators to answer, building an adventure spine and weaving in people and events to make it all interesting. It makes for interesting and inspiring reading, and could prove useful for anyone planning post-apocalyptic adventures, even if outside of this particular game system (indeed this whole book would make good background reading!)
Finally, The Decaying Earth lays out a timescale for the collapse of civilisation as we know it. It can help you determine the state of affairs right now in your game, and provide a roadmap for what's going to happen down the line. It's unlikely that the Investigators will make much difference as nature reclaims everything. There's also a chart to help you determine how difficult it is to find things as time goes on. It may be surprising to see how hard it will be to find books... there again, they are made of paper and if you are short on fuel to stay warm or to cook, they may get repurposed.
Overall, if you want to bring the world as we know it to an end in your game, this is a very good manual. Of most use, of course, if you play Trail of Cthulhu but there are enough useful ideas and concepts that I'd recommend it whatever ruleset you run your post-apocalyptic world under. It looks at a wide range of considerations without getting bogged down in trivial detail, and makes for a thought-provoking read.