See also Deep Secret (spoilers).
He has the help of the ghost of his own mentor, Stan Churning, who is haunting his car and listening to Scarlatti. He has a list of 5 possible candidates but none of them sound very promising. He has a hightly frustrating experience when attempting to meet the most likely, Maree Mallory. He crosses her off the list and arranges to meet the rest at a science fiction convention in a labyrinthine hotel complex.
Although magically excluded, Maree shows up, attending the con with her uncle, sci-fi author Ted Mallory, his unpleasant wife Janine Mallory, and her younger cousin Nick Mallory. All the madness of a sci-fi convention combines with magical misbehaviour to cause a truly chaotic job for Rupert who has more on his hands with the candidates than he bargained for.