When the head of England’s Jesuits, Henry Gamet, is tortured to reveal the names of his fellow Catholic Gunpowder-plotters in Frank McGuinness’s Speaking Like Magpies, his stance (on a chair, in a grubby shift, outstretched arms trailing wires) manages to suggest not just the Roman Empire’s execution of the Christ Garnet serves, but more recent images of abuse in the Middle East. This is the one point at which the director, Rupert Goold, makes overt the similarities between seventeenth-century and contemporary attitudes to “foreign” religions, seditious clerics and amateur bomb-makers. For the most part, McGuinness’s play keeps in focus the specific conflict which helped forge Britain’s peculiar compromise between Scots-Calvinist democratic instincts and French-Catholic absolutism when James I turned his back on England’s recusant families.

For McGuinness, brought up close to the Irish border, the mixture of toleration and anti-Popery that defined Britain for several subsequent centuries is a live issue. His range to date is impressive and rarely focuses, unlike the work of most of his compatriots, on the internal state of the modern Republic: Someone Who’ll Watch Over Me, for example, explores Irish and English hostages at the mercy of a different kind of zealot, and his best known work, Observe the Sons of Ulster Marching to the Somme, probes the interstices between Ulster Protestantism and British patriotism. The latter play dwelt on the personal consequences of cultural inheritance, but McGuinness’s staging of the foiled 1605 conspiracy is, by comparison, wilfully “Jacobean”. It combines complex political disquisition with flamboyant morality play, and its cast of stock characters includes a preening king, a goading devil, a Bible-brandishing Puritan divine, a mock Pope out of some post-Reformation nightmare, and a coven of hooded, Gothic conspirators with their instruments of death and medieval “guild” banners.

The interlocutor who gives voice to Father Gamet’s troubled conscience, a sprightly Kevin Harvey in goatskin trousers and tiny horns, is referred to as the Equivocator. Gamet himself, played here with humane ineffectuality by Fred Ridgeway, is credited with promulgating the doctrine of equivocation - which allows suspected Catholics to mislead their interrogators without actually lying - and is often held to be the “equivocator” Macbeth’s Porter welcomes into Hell. Harvey’s witty “devil’s advocate” is also our key to James’s complex motives, but it is left to Gamet to argue the ethics of mass murder with the conspirators Catesby and Wintour, a frenetically unpleasant double-act (the chattering magpies, perhaps) who challenge Gamet’s repeated failure to champion the right of the oppressed to resort to violence, but who also cruelly bait May, a maidservant to Catesby’s cousin and Garnet’s friend and protector, Lady Anne Vaux. Played by Ishia Bennison as a model of Christian integrity in adversity, Anne is still there in the last scenes, after much intervening mayhem, hoping to reclaim James’s now distant queen for the old faith. The young May will fall victim, like Gamet, to postbomb-plot outrage and will die merely, in her case, for having known the conspirators.

Fireworks are dispensed with early, and the crucial moment is represented by a quickly snufled taper. But the piece’s real pyrotechnics are in language, imagery and argument. Working in broad symbols for much of the time, McGuinness uses verbal and visual oppositions between English suavity and seeming Scots incomprehensibility, between Catesby’s beauty and the unfortunate countenance of James’s spyinaster Robert Cecil, between Papist finery and the sober attire of Reformed clergy, between the light in Anne Vaux’s garden and the conspiratorial Masque of Darkness.

Gamet’s problem is, ultimately, that although he refuses the plotters forgiveness (a priest cannot absolve murder as-yet-unfe. nunitted, the sinner patently lacking both penitence and the “firm purpose of amendment”), he has nonetheless heard their intentions “in confession”, and is therefore bound to secrecy. Ile authorities know that Gamet and other loyal Catholics hate terrorism “as much as we do”, but Gamet’s refusal to turn King’s evidence still leaves him open to accusations of complicity and casts on his faith community the suspicion of political sedition.

In a deeply troubling work (buoyant wit and dazzling stagecraft notwithstanding), the playwright explores two simple moral conundrums. Garnet must struggle with the lengths to which a member of a peace-loving faith may go to protest against, or eliminate, oppression of co-religionists. And as the Equivocator plays tellingly on the internal calculations of William Houston’s easily flattered new king (who arrives with a natural bent towards toleration, if only because of his political inheritance), we share, too, in the moral turmoil of a head of state propelled by a combination of spycatcher manipulation, genuine security threat and personal fear towards the wholesale repression of dissent.