Merlin the wizard is a good-looking, but very well-disguised woman. She's also ambitious and becomes Arthur's mentor, falling in love with him. It's reciprocated, and they elope, leaving the wizard's cave to Tertia, Unita, and Marlene (Merlin's apprentices and her sister).

They start a detective agency, however bored with the lack of fifth century crime, they start up the Temporal Detective Agency and use Merlin's time portal to move from Camelot to the 21st century, where the cases are better. They also take Galahad, now owner of the Ole Grill restaurant, so he can act as their front.

One morning, the portal transports Tertia onto Nelson's column in Trafalgar Square, and then on a temporal diversion to South Wales in 1734, where she joins up with Bryn, a young Welshman, and sees her deadliest enemy - the Black Knight - who she believes to be Bryn's father.

Tertia and the agency try to solve the thefts of the Koh-i-noor diamond and the Mona Lisa. Realising that South Wales and the Black Knight are very much connected with their investigation, Tertia and Unita relocate to South Wales in 1734, where they come up against a gang of vicious ship wreckers, led by their deadliest enemy, the evil Black Knight.

Indeed, within an hour of arriving, Tertia is nearly killed by him. Going undercover, Tertia becomes a teacher in the village school, while Unita becomes a maid in Bryn's father's household.

One stormy night, they're on the clifftop, looking for wreckers, when they're surrounded. Help comes from Sir Gawain, who now lives in the South Wales village as the local squire. He had followed the Black Knight through time to Wales where they continued their fight. Gawain is actually Bryn's father, and has even more reason to hate his enemy after he believes the Black Knight killed his wife.

After convincing the Black Knight that she wants to join his gang of wreckers, Tertia learns where they intend striking next and helps Gawain defeat his enemy and his wrecking thugs in an ambush.