For this roleplay, I would be driving through a Rural area, have some car trouble, or just be lost. I would stop at the closest building that I could find, a private psyc hospital. Unknown to me, one of the patents, whos parents pay a lot of money to keep her there, but rarely visit,accidentally dies.
With me being physically identical to her, the staff, especially the head doctor, would conspire to lace my drink, and replace her with me, so as to keep the money coming in. Her mental condition could have her prefering to be in some themed outfit, schoolgirl, maid, or fetish attire.
That, or I could be given a laced drink, and end up as the staff's pet,puppygirl or ponygirl, Im easy.