The player characters are a cadre of high-profile thieves, akin to Ocean’s Eleven or whatever other heist movie you liked. The group, who has never worked before but each has impressive credentials in the underworld, are united by the bond of stealing the precious stone currently under guard at some high-profile building in the city.
Have each player create a character, making sure that they all work well in a group, with niches that will be fun and interesting to play. Then, before sitting down to the first game, take each player aside, seperately, and explain to each them that their character isn’t ACTUALLY a thief, but a government agent trying to bust the other players after the diamond is stolen. Use email to handle the characters’ double agent stuff, outside of game. Impress upon them the requirement of secrecy, and tell them that you’ll allow the others to kill his character if they slip up. In the opening, their mysterious contact informs them that they have been infiltrated, but has no idea who it is. Offer a reward for anyone whose role as a mole is discovered in the form of XP or whatever reward Player Characters receive in your campaign
Then, sit back and watch the fun.
