So, you've stepped up and offered to run the Delta Green RPG for your group. Here's a massive sourcebook to help you make the world of the game come to horrifying life for the characters... It includes a history of the alternate Earth in which the game is set, loads about the Mythos, rules for new threats, a good understanding of Delta Green as an organisation, and much more; and ideas for adventures are scattered freely throughout as well as there being a complete scenario ready for you to run.
The Introduction begins by recommending that you go and read the Delta Green RPG Agent's Handbook first. Then it reminds you that first and foremost, Delta Green is a horror game dealing with people finding out just how insignificant human beings are in the universal scheme of things, with the end of humanity itself - and probably going clean off their heads if they aren't killed in the process. It's about conspiracies to keep the horror hidden - some concocted by the horrors themselves to keep us human beings in the dark, some dreamed up by well-meaning folk who want to protect humanity from knowledge that can quite literally drive them insane. It's about humanity's constant struggle to understand things beyond all knowing, to defeat threats that are far too strong and will ultimately destroy them... it's about the struggle, not about the end. We then find out that Delta Green's been running since 1942, born out of the need to deal with Nazi experiments in the occult during World War 2, but by the present day there are TWO Delta Greens, one an unsanctioned conspiracy and one a very deep and dark government operation: both, of course, think they are the real thing - and the one the player-characters work for probably think it's the only one. This section then talks about what Delta Green agents do - and what the Handler does in running the game. Never mind the horrors that they face, Delta Green itself should remain a mystery, with the party only ever being told what they need to know, if that.
Next, The Past delves deep into the (alternate) history of the world and of Delta Green itself, going back to the 1920s - tying in neatly with Lovecraft's writings about a township called Innsmouth in Massachusetts. The amount of background is tremendous and fascinating - but the vast majority is for your eyes only. Your players won't find it out, not even during the course of the game... unless perhaps you choose to run a campaign set during this history rather than in the present day. This history is massive and detailed, often several entries per month for each year from 1928 until May 2017... not to mention sidebars and notes on all manner of stuff.
Then comes The Unnatural, where hidden knowledge unguessed at by even the most experienced Delta Green agent is revealed. Part of this is due to the deleterious effect such knowledge has on the human mind: Delta Green compartmentalises the information that it does keep, and much is destroyed. There's no library of rituals and spells to consult, for example. Here, though, we read of tomes of occult lore, unnatural - perhaps alien - creatures, and more mindbending stuff. Fundamentally, much can never be understood. It's just too weird for the human mind to cope with. There are plenty of unnatural threats here to throw at the party, ideas for plots spawn as you read through their background notes and histories. There's a collection of sample tomes, most of which should be read with caution if they are read at all, and deep discussion of rituals and 'hypergeometry' for those wishing to risk bending reality as well as their own minds. And if the array of unnatural entities paraded for your use is not enough, there are detailed instructions for designing your own.
The next chapter is entitled The Schism. Here is the secret history of Delta Green, and an explanation of why there are two groups who both call themselves Delta Green. The advice as to what to say to your players is simple. Tell them nothing. Of course, you may decide that it's nothing like what is described here. It's your game, after all. There are details of how agents are recruited - this has either already happened to the party or you may decide to play it out with each of them in turn. There are also notes on how typical (if there is such a thing) Delta Green missions run, equipment available, and much more including several senior Delta Green members who might turn up at some point during your game. Both groups claiming to be Delta Green are covered in parallel. Take your pick. This section ends with notes on what Delta Green (either sort) actually knows about matters unnatural. Let's just say it is a very short section.
Now we come to The Opera. Derived from the Delta Green slang for an operation - 'A Night at the Opera' - it explains what goes into an operation: how to design it, how to run it... and even how to expand it into a whole campaign. Plenty of advice here as you put your plans and plots into motion. Learn how to construct the events and clues that will populate them... and to embue the whole with an air of horror, engendering fear and the awful knowledge that they're not in control into the party.
Finally, there's an adventure to help you put everything into practice. Operation FULMINATE: The Sentinels of Twilight tells a tale of disappearances from national parks, and why not everything that's lost ought to be found. Yet it has been...
Equipping you with a wealth of information and ideas to run a Delta Green RPG game, this is an invaluable resource to read and study again and again. Your game will be immeasurably better for it.