Dastardly, I know.
See, I told them that their characters were joining some shadowy government agency (Agency 17) that's so deep under wraps that barely anyone knows it exists. In reality, the player characters are all part of a shadowy criminal counterintelligence organization that has been raising all kinds of hell, all while convincing their operatives that they are legitimate.
Now, just to be clear, Agency 17 is the criminal counterintelligence agency.
Section 17 is the legitimate government organization that Agency 17 is mimicking.
Every mission the players go on will have a twist somewhere that turns what they're doing from doing good to doing bad. It's about making sure that they perceive in game events one way, while something else entirely is going on.
The first mission involved someone who was actually a legitimate NSA agent, who the players were mislead into believing was simply a journalist. They never did figure out he was NSA, instead believing that he had a source within the NSA who he used to feed info to his uncle.
This current mission, the drug hijacking, has the twist further down the line. There may be legitimate reasons for attempting to steal twenty tons of cocaine from the Los Zetas cartel, but Agency 17's motives are definitely not pure. The players really have no way of knowing this except for the effects of Agency 17's use of the drugs. Possibilities include selling it for profit, planting it on individuals to frame them, and possibly even increasing Zeta/Sinaloa violence in Mexico.
Other times, I hope to hide what the players are actually doing. An example would be having them retrieve black boxes from a downed Agency 17 craft to prevent legitimate investigative crews from being able to access them. Another could be planting devices that don't do what the Agency tells them they do- a 'keylogger' could really be something more insidious (stuxnet comes to mind), while "medical supplies" could be crates full of guns and 'oxygen tanks' hold deadly sarin gas.
Now, there are some important things to consider when a major facet of your campaign involves lying to your players consistently:
- Especially in a system like GURPS where characters made for one campaign will suck if dropped into another campaign/genre, it's important to ensure that people's characters will be useful throughout even after the reveal. For Agency 17, learning that their employers are evil won't serve to change the fundamental understanding of the universe (no magic, no time travel, no aliens), it's just a large betrayal of trust.
- If your players begin to see the cracks, let them! How they decide to handle the small inconsistencies will help force how and when the reveal happens. There's also the fringe benefit that things that don't make sense to players due to fridge logic can later be explained as a consequence of their characters constantly being mislead.
- Controlling information becomes hugely important. Giving out too much information will blow the surprise early. Giving too little means that the players will be entirely clueless. I've tried to make them work for the really juicy bits, and I've accepted that they won't always learn everything.
- A lot of this rides on the players accepting things at face value and not digging. If their handler says that those crates are medical supplies, hopefully they believe him and don't try to smuggle in a few pistols on the top themselves.
- It helps that Delusions exist in GURPS which can help explain why their handler never caught on that their employers were scum.
- Recovering the black box of a downed Agency 17 plane to prevent other organizations from using it
- Moving something (Food, Medical Supplies, Building Schematics) that is really something else (Drugs, Sarin, Weapon Schematics)
- Protecting wanted terrorists, fugitives, assassins, etc
- 'Enemy agents' are really legitimate CIA/SAS/DEA/Mossad
- Framing CEO/Dignitary for Child Porn Possession