The scenario takes place in a general office somewhere, probably one belonging to a small professional partnership like a solicitors. The Solicitor's assistant had an unexpected accident and broke her leg, and they have hired a temp for a few weeks. However, they are now having problems with missing files and leaked information. This sabotage must have come from someone inside the organisation....
You can introduce characters into this in any of the main roles: most obviously the Solicitor, the Temp or the remaining secretary. However there are a few catches....
The Solicitor: A professional, probably a solicitor, accountant or architect.There are obviously places for other characters in this - as a partnership there will be other professionals affected by the sabotage. There will also be cleaners, secretaries and other support staff as suspects. Another useful role is that of a friend outside the business, who knows both the Manager and the Spy and is friends with both of them. The character then has to decide whether to warn the manager that they have a spy on the staff. This works best if the friend, the spy and the manager are all PC's.
This is entirely up to the GM, and depends heavily on the players' actions. It is up to the GM who they want to make responsible for the sabotage, though making it one of the three principle characters could be considered a little obvious. The spy may, if they are not responsible, find themselves forced to track down the saboteur themselves in order to make the company lower security so they can get the documents they need.
This is not designed to be a long and complex scenario though it can be played that way. It works well for one evening's game, as long as you ensure that there are always a large number of people who could be responsible for the sabtoage. You can stretch the game out by delaying the characters' elimination of suspects. In general most of the action in the game comes from character interaction, and there is unlikely to be much, or even any, combat. The other aspect of this game is that it is normally best to encourage the players to pass you notes, stating their actions. The aura of paranoia this creates works for the game - especially if the spy and the solicitor are both PCs and are trying to catch the saboteur while also trying to outwit the other player.