This plot revolves around a low-level female PC. There can be other members in the party, but this plot focuses on her.
BACKGROUND: The female PC believes she is the daughter of some low-class rural folk in a small country. She is aware that she bears a resemblance to the current queen who is married to a young king. The queen disappears, believed kidnapped, and forces are mobilised to try to find the missing queen.
The PC, while adventuring in other lands, gets attacked several times by assassins (more than several; more like many), and discovers from one of them that they were hired to kill the queen by some of the nobles. This should happen a number of times, making the PC and her companions very annoyed. She is sure that she definitely isn't the queen. In an attempt to find out what's going on, she returns to her home country (still under attack from assassins).
There she is discovered by the local Thieves' Guild, and they fill her in on some of the details; namely: she is actually the queen, but was mind wiped by the king's Grand Vizier under orders from the king, after she discovered some of the king's nefarious dealings. She was given the memories of a rural woman with an uncanny resemblance to her, and switched with her to keep her in a safe place (ie. out in the country). Later he decided to kill her, so as to bring suspicion of treason upon some of the more powerful nobles, giving him the chance to eliminate them. The PC accidentally escaped though, and found herself adventuring, while the rural woman's body has now been found, and all believe it to be the queen, except some nobles, who want to kill the PC so as to be able to present two bodies and thwart the king's plot. The Thieves' Guild offer to help because of the instability the state of the country is causing to them.
They can smuggle the PC and her companions into the palace, where she can confront the king and his crony. During the fight (or whatever develops), the Vizier should get killed, to prevent him reversing the mind wipe. If the king is killed, the throne passes to her, or alternatively to the next male heir (GM's choice).