Solitary mystery writer Julian Wilkes doesn't want a pet, but his sister persuades him to visit Lingwood Animal Rescue, where he is immediately taken with a large ginger tabby cat. Before he can settle into the joys of cat ownership, however, he discovers something very unusual about his new companion.

Macavity Birch is cursed. By day he is a large tabby cat. At night he can be himself—a human male with ginger hair and oddly yellow eyes. He didn't mean to end up in the animal rescue, but he never meant any harm when playing the prank that resulted in his curse, either. Happily, Julian adopts him. But while exploring his host's home, he discovers the diary of a long-dead relative.

Unfortunately, not all of Mac's ancestors are dead and buried. His great-great-great-grandmother is very much alive, and she's a powerful witch who doesn't take kindly to the sharing of family secrets. When Mac reveals himself to Julian in order to save him from bigger trouble, he achieves just the opposite, plunging Julian deeper into a magical mystery with him.