The exorcist and his or her partner, sometime lover and long-time friend receive a request from the ruler to handle a delicate situation.
The exorcist accepts with some concern. Ever since returning from the funeral of a relative, the exorcist's partner has been behaving out of character. Though feeling foolishly paranoid, the exorcist performed several tests to reveal possession but nothing showed up.
They accompany the messenger to a remote villa where the ruler's sister greets them. They are led to the heir apparent. A possession is clear even without tests. But that's the last thing that is clear.