The opening of Paul Webb's Four Nights in Knaresborough is promising: the lights pick out from the darkness four chain-mailed figures who surround a priest praying at the altar. As the compliant clergyman murmurs an Amen, the knights seize and murder him. This act is deeply symbolic, encapsulating the conflicts of this drama about church versus state, the law of the land versus the morals of the individual.

It's disappointing, then, to find that the script as a whole is shallow and curiously teenage. Although Webb introduces a potent and intriguing historical situation - the murder of Thomas à Becket in Canterbury Cathedral by Henry II's knights - the action deviates from this to focus instead on the emotional reactions of the four knights, who, having fled the wrath of the people, are indefinitely walled up together in Knaresborough castle.

As such, the unfolding action functions on a superficial level, emphasising clashes of emotions and personalities, and glossing over the deeper political, historical and psychological consequences of the murder. This is a shame, not just because the play fails to bite any of these achingly pertinent ideas to the core, but because, despite its very modern viewpoint, it also fails to match the thought-provoking precedents set by other writers who've approached the same topic.

A clutch of strong, engaging characters contrasts with the vagaries of the set-up, and the players manage fully to capture the coarseness and brutality of the knights and their military life. Moments of high drama contrast with sitcom-style humour.

Nick Moran (pictured) is a laddish Brito, full of confidence in himself and his animal attraction, but only gradually aware of his real needs, and those of his fellow inmates. The troubled de Traci, played by Robert Cavanah, finds he's imprisoned with the object of his affections, while Morville (Joseph Millson) is driven towards rather subdued madness. Meanwhile, Fitz (Tim Dantay) and Catherine (Joy Brook) vie for the position of Most Haunted Character.

Director Paul Miller makes excellent use of the performance space, capitalising on Hayden Griffin's minimalist designs and strong colours and images to lead the players out of darkness, and Matthew Scott's soundtrack, although clichéd, nonetheless reflects the bleakness of both the age and the drama.

But in the end, the skills of the cast and production team are restricted by an ineffectual script, and the audience is left to draw its own conclusions about ideas such as diplomacy, war and political power, which are there, somewhere beneath the surface.

- Olivia Rowland (reviewed at the Cambridge Arts Theatre