The players are hired to retrieve a special artifact or treasure from a distant locale. They are meant to meet a contact or spy there, who will direct them further in their quest. Evil parties have intercepted the spy, and replaced him/her with their own, intending to deceive the players.
The players could end up unwillingly working for the enemy - but for how long? Will they realise before stealing from, attacking, or even murdering the innocent? Maybe the players realise, then counter-double-cross the evil parties!
What will they discover if the artifact is recovered? Will they finish the job and hand it into their employer, or does the item hold secrets that lead further into adventure?