Shirley McKay's latest in the Hew Cullan mystery series, set in 16th-century St Andrews, is not perhaps the strongest of the four novels, but fans of hers will be delighted to see how she uses this particular story to set up an extremely intriguing future for her scholarly hero, possibly in the pay of that creepiest and deadliest of Tudor spymasters, Elizabeth I's Francis Walsingham.

It's a necessary broadening of scope for McKay, for while her detailed descriptions have deepened her historical thrillers beyond the average of the genre, they have tended on occasion to narrow them too.

This narrowing makes the plot trickier to follow in its denser parts, and slower to get going. As always, she shows a splendid ear for language, conjuring up the times with a mere well-chosen phrase or two. In this novel, Hew is called upon to investigate when a tree in the university grounds appears to weep blood; Andrew Melville, the controversial head of St Mary's College, is being besieged by anonymous threatening letters; and, behind his back, a young student who blames Huw for the loss of his father is misinforming on him about his relationship with the married sister of George Buchanan, one-time tutor to James VI.

Much of this can feel like small-town concerns, especially in the early chapters. The suspicions and gossip and spite of a small place where everybody knows everybody else's business take precedence, which means that the threat that lingers rather softly in the air doesn't hit home hard until a soldier guarding Archbishop Patrick Adamson's residence is killed. A fellow soldier, a young man from Orkney who has struggled to fit in and who has been befriended by Huw's sister Meg, is fingered for the murder, and Meg feels obliged to help him, at much cost to her own safety and that of her family.

Is there a connection between this death and the sinister letters targeting Melville, the Archbishop's dubious practices (he is seriously ill but that hasn't stopped him having carnal knowledge of a local "physick's wife", who cures people with herbs and potions and which leaves her open to charges of witch-craft)?

The novel takes its time to reach this question, winding its way through the dark and ghostly streets of the town that threaten to obscure it altogether at points.

What changes the novel considerably, though, and suggests a possible new direction for McKay herself, is the arrival of James VI. McKay's depiction of this lonely 17-year-old king, isolated and virtually kept prisoner, paranoid and passionate, deprived of his friends and his supporters, is superb, and made me wish he had appeared much earlier and was given a greater role. It may be unfair that the fate of kings and queens matters more than the fate of ordinary folk, but that is because they have so much more to lose. There is a reason why writers like Philippa Gregory and Hilary Mantel focus on the fortunes of aristocratic leaders and those Machiavellian power-makers close to them: what is at stake is enormous. The personalities that drive the fortunes of a nation are fascinating to us, and McKay has one of the most unusual ones in her hands.

We also like to see power at its most absolute and its most corrupt. The character of the young James embodies these extremes and McKay, in the glimpses she gives of him, shows a new understanding of psychological complexity in her portrayal here. Generally she has eschewed psychological depictions of her characters, preferring to demonstrate their moral codes by their actions (one of the reasons that allegations of Huw's affair with Claire Buchanan are so readily dismissed is because he has lived so closely and so long by his principles).

But with the introduction of Walsingham, and the presence of a contradictory young king, who is both entitled to the throne and yet not entitled to it, McKay has entered a more challenging area. The beginning of the novel doesn't suggest that Huw himself has much more to give us; the ending suggests something different.