This supplement is for those who'd like to bring real horror to their Spycraft 2.0 game. The main thrust of the book is horror of the supernatural nature, perhaps you want to mix in a bit of the Cthulhu mythos or other such monsters from beyond, but much could be utilised if you are seeking gritty realism, the sort of fear and horror that quite turns the mind and which can be imagined as part of a real-world espionage story.
To start with there are a selection of campaign qualities with which to set the tone that you want. These determine which aspects of the rules presented here you will use. If you want characters who are scared of the consequences of the sort of things they will have to do, consider using the basic classes from Back to Basics - they are less capable, and can be used to generate the feeling of 'I could die at any moment'... just the kind of thought no spy wants to have when embarking on an operation! Or you may wish to mix in occult knowledge and spellcasting for a quite different kind of horror.
To model the toll that such fear takes on characters, an expanded set of rules for stress damage are presented, making minds as vulnerable (if not more so) than bodies. This causes characters to sustain mental damage - in terms of phobias, mental disorders and even catatonia - as a result of the horrors to which they are exposed. Care needs to be taken here, most players find it far easier to cope with their characters sustaining physical injury (it's only hit points on the character sheet after all, they do not feel the pain) than anything that messes with their minds. To be run successfully, players and GC need to be in accord: even more so once a character sustains lasting mental damage which has to be role-played, it is not something purely represented in game mechanics.
Especially effective when created as a collaborative effort between GC and player, an Unhinged Subplot can be developed and run over the next few sessions culminating (you hope!) in a resolution that will help the character regain his sanity in a manner that also contributes to the ongoing storyline.
A whole slew of things which can cause a character to get stressed out are listed. Many assumes a rather less heroic approach than the classic spy/thriller one, casting the characters as mere ordinary folk rather than the larger than life heroes that normally inhabit a game, especially one like Spycraft. Then the discussion moves on to Forbidden Knowledge - arcane secrets it is probably best not to delve into... only you know how curious characters are, and they may need this knowledge to defeat the threat that they are up against.
Throughout, there are examples and suggestions of ways in which to incorporated different kinds of horror into your game. There is also a collection of monsters and cosmic horrors, if that is the direction you wish to take.
This style of game will not be for everyone, and other groups may prefer to switch to a different game system that's built around such horror from the ground up... yet if you like the spy genre and Spycraft in particular the way that these rules work embeds the horror into the core game mechanics rather than bolting it on as an afterthought.