Maybe I'll tell you my idea first.
The princess of the ______ kingdom has had to flee from her home because of an attack from another kingdom. As the war continues to rage on, she's left to wander the deserts, soon finding herself being followed by a wandered.. it seems. In need of protection (as a girl shouldn't wander around by herself, especially a princess) she begins her travel with the stranger.. they share experiences, memories, conversations and then, she tells him of her royalty.
She awakes one night to find him ready to plunge a dagger into her.
This man was the assassin, sent to kill what was left of the royal bloodline, and yet he hadn't know this woman was her.
Complications arise as the two are left stranded together.. ect.
All I could think of.
Though I'd like to hear your idea.