As I mentioned conspiracies the other day, I figured I'd share a trick for constructing them which makes them easy to construct yet consistent enough to maintain a "The Truth Is Out There" vibe.
Start off with a secret - If your familiar with the Shock RPG, consider this a single shock. It's some thing or event which changes everything and upends all previous assumptions. Good examples include Aliens having landed, people developing psychic powers, time travel being possible, demons being real or something like that. If it's a big enough deal that it could be a premise for a game all by itself, that's probably a good secret.
Write it down in the middle of a piece of paper or on it's own index card. That's your premise, now comes the tricky part. Think of three or four new secrets that derive from that first one. They might be extrapolation (If demons are real, then maybe some magic is real and vampires are real). They might be consequence (Telepaths run the spy agencies and telekinetics control gambling and sports). They might be limitations (it's only possible to view through time, or it's only possible to travel with organic material). Whatever. Come up with a handful of these.
These seed secrets are your real baseline. You want to use each one as the truth that you build a conspiracy around. Based on this seed, some group is doing something. Vampires hunt in the shadows, treasure hunters are searching through time for secrets and so on.
Once you've got a spread, pick one of them and present it to the players as "the secret" and construct a game around that. You'll go into play with a handful of related secrets and a truth that players can eventually dig down to reach.
Given that, here are three more tweaks,
First, as proposed, I imagine the GM is doing this in secret because the players want to be surprised. This is not the only way to do it. If the group likes having meta-knowledge, then they may want to be in on it. Alternately, if the group wants more of a "Faction War" kind of game (like Feng Shui) this can do the job.
Second, complicated secrets can be cool, and sometimes a seed secret is interesting enough that you might want to make it the central secret. As an example, in the Terminator franchise, the base secret might be that Time Travel is possibe, but the real secret is that there's an AI in the future sending machines back in time to try to keep itself from being destroyed.
Third, which is related - you can actually repeat this process as many times as you like to create more complicated secrets. If you do this, not every secret needs to be its own conspiracy. Some may be shared secrets or otherwise part of the landscape (like time travel in Terminator) that may need to be discovered, but not necessarily unraveled.