Heather's home is the grand old historical estate of Castlemaine, where her parents are caretakers. This is all very well, except in summer, when the place is overrun by tourists. Desperately seeking a tourist free hiding spot, Heather retreats to the edge of the estate, to the strange little mound said to be the grave of Wild Robert, who lived and died most mysteriously 350 years ago. In her frustration, Heather angrily calls to Wild Robert to do something about the tourists, and to her very great (and understandable) surprise, he appears before her, a handsome young man in period clothing, somehow brighter than everything around him, but very much corporally present (so not a ghost).
Wild Robert was a magician, and he still has considerable powers of magic. During the course of his day with Heather, he works considerable magic and mischief. But though Heather is consternated by tourists being turned into sheep and the like, as she learns more of his story, and empathizes with his reactions to seeing his home 350 years in the future (this is the part that makes this timeslip in my mind), her primary response is one of deep sympathy. And the book ends with Robert returning underground, and Heather wondering if his heart can ever be returned to him....
And I really wonder that too, and want the next story really badly! It is Heather's empathy for Robert, and the sadness of his story with its twisty magic and familial betrayal, that gives the book more weight than it would have it were just simply magical high-jinx worked on tourists (although that makes for fun reading in its own right!). But because the story ends after that one day, we never get to explore that deeper part enough to make this a truly outstanding book.