Raiders
Most advenurers have been forced to do a stint as caravan guards at some point in their career, just to see them through hard times. One would hope that they'd learned enough from the experience to pull off a successful raid themselves. The only catch - they mustn't kill anybody.
Quite a simple plot, but one that could be fitted in to almost any campaign, particularly one with a diplomatic or intrigue element.
Introduction
A noble of the kingdom is suspected of having stolen state secrets, crucial to the defence of the kingdom. There is, however, no proof and the noble is sufficiently powerful that the government cannot have him seized and interrogated upon suspicion alone. The noble is currently travelling towards Esker, a medium sized city near the border of the kingdom - it is thought that he may have a contact here who he will pass the secrets too, and it is thus imperative that he not be allowed to reach Esker. The noble is travelling completely openly and has joined a merchant caravan (they are travelling together through a wilderness region for added protection from bandits). (Naturally, none of the above is common knowledge in the kingdom.)
The plot
The PCs are hired by the government to abduct the noble from the caravan, disguised as bandits. He can then be delivered to the government who will interrogate him, whilst making it appear that he has been seized by bandits for ransom. The catch is, of course, that the PCs must try not to kill anyone. This is not an order, but if your PCs are the sort who are being hired by the government, their morality will be such that slaughtering a group of merchants and merchants' guards should be repungent to them. Destroying property as a diversion (such as starting fires) is a little better, but ruining someone's livelihood is also not a very good thing to do. If they do accidentally kill people they should certainly feel appropriate remorse and act upon it in the future.
The PCs must also not be seen clearly. If they are, then this may cause serious problems: the merchant might, for example, post a reward notice for them. There is also the difficulty that a lot of the more intelligent things they could do (in terms of carrying out the abduction) are extremely morally dubious: for example, if they aided some real bandits to attack the caravan, the bandits would probably steal and kill at will. There could also be problems if the bandits refused to give up the noble (wanting to really ransom him).
Details of the caravan
Feel free to change these - they are merely suggestions.
16-20 wagons including:
Some carrying various goods of wood and metal.
Some carrying food.
Some carrying fine silk or some other material that will easily burn.
One carrying barrels of naptha (just to make things interesting).
Two of families travelling with children.
The merchant's wagon.
Two or three wagons belonging to the noble: e.g. one for him, one for his servants, one for his guards.
A couple of guard's wagons.
Number of guards (including the merchant's and the noble's) could be around 20. There might well be a few good warriors (for example the noble's personal guard) (and the noble), but most wouldn't be that good. Wizards and clerics appropriate to the magic level of your world. Definitely guard dogs, which should be able to smell any stealthy raiders.
Final notes
The merchant is canny enough that if the PCs ask to stay the night with the caravan he will make them give up their weapons and will guard them. He is in bandit country after all. However, visiting the caravan could be wise to gain information.
The PCs must abduct the noble, silence him, and deliver him to the government (not too far away).
Don't be afraid to play on the consequences - e.g. to do with morality if they kill people (they might want to try to secretly find out who that person's family are and give them money to make amends), or consequences if they are spotted.
If you are feeling cruel, you could always make the noble innocent of treason. Then, when he is released, the PCs will have made a powerful enemy.
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? Responses (10)

I like this: it has a moral dimension which many plots lack and it would require some considerable ingenuity on the part of the PCs to succeed.
ephe!

And all those possible twists, with traitors capturing the faithful noble...
Has interesting potential.

To add a twist, they could find out their Mr. Smith (Government type) was not really a government person, but only presented forged credentials and an office that appeared to be part of the local buracracy.
Such a revelation should be after they deliver the noble, but before they get paid.
Now what do they do?
Get the guy back?
Try to clear their name?
Who will believe them?
If they discover it before the nobel is traded, now they have this noble with them who may or may not trust them.
Say thank you Sneakers for this wonderful plot twist.

One more thing...
Maybe the caravan is a cover for the forces of the government trying to move this traitor. All of a suden, secretive warriors are involved in the fight, or may chase the players after they do the snatch.

Moonhunter, some of those twists are really nasty! And thus very fun. If the man who hired them had only had forged credentials, as well as the complications you mentioned there would be the added fun of trying to figure out who he was. An agent of another country? Part of a sophisticated organised crime outfit? If they do try to take the noble back, they could start getting in all kinds of trouble (in addition to the trouble they are already in).

Boy, this one is just a great thing to play with.
Another twist perhaps. Have another set of kidnappers beat the players to it. These kidnappers are a third entity actually after the defense secrets in hopes of bribing the enemy, the kingdom, or both.

I came across this in random sub and while I think it is a solid idea, I think this has been developed too little and offer nothing more than the idea.
But to sum up what people listed as the positive points:
Potential Moral Problems:
I think this could be improved upon if you place some NPCs in the caravan with which the PCs have some meaningful connections. This would increase their motivation to be subtle and use as little force as possible.
Also what if the PC don't know exactly what the Noble looks like, and the Noble is in disguise. They could grab the wrong guy, and then if the PCs were also tasked with getting the information out of him they would have to determine if the Noble is just playing dumb or if they really grabbed the wrong guy. The grabbing the wrong guy thing could implemented if there was some marker, such as horse or a medallion or something that Noble passed off to another NPC or the other NPC just had the back luck to own. Worse case scenario they get the peasent to confess to being a Noble traitor and the real Noble traitor gets away.
twist: Again start with they capture a guy who swears left and right that he is not noble X. They use telepathay or what ever and determine that this guy possess the mind of a simple cobbler. You could even toss in a few confessions of crime unrelated to the one they are after. The twist is that this really is the Noble and he has just cast some sort of charm on himself to make himself believe that he is a simple cobbler. A version of this could also be implemented with the same set up, but instead of charming himself, there is a wizard from the other nation who has been controlling the noble. This wizard is traveling with the caravan, and is a real threat to the the caravan's and the nation's safety. If the PC discover this they have to then have to attack the caravan again to protect it form the wizard...who as far as the caravan is concerned is just another merchant they are sworn to protect.
twist: What if the state secrete is something that no memeber of the kingdom should know? For example the prosperity and ricness of this kingdom is based on some evil-blood pact with a dark power. Lets say long ago an ancestor of the king agreed to give the body and soul of twenty men and women at the age of twenty to be eternally tortured and bound to a dark demon. In exchange the kingdom is free of plagues, there are not wars within its borders, crops never fail, women are tall and men are good looking. The noble is gathering his case and plans to expose the kings evil pact. As collateral the demons holds the rights to the body and soul of every innocent within the kingdom...every child, savant or pure of heart. The noble is a real stick in the mud when it comes to demons and plans to break this contract. The 20 sacrifical men and women are being transported with the caravan, ignorant of their fate. Or something like that