As debut plays go, Giant has some very experienced hands behind it. Directed by Nicholas Hytner, who runs the Bridge Theatre, and written by Mark Rosenblatt, a director for more than two decades, it sounds like cheating to call it a debut although it is indeed Rosenblatt’s first foray into writing for the stage.

You would not know it from a slowly brilliant first act, stupendously performed by its cast, which mixes fact with fiction in its dramatisation of a scandalous moment in the life of the children’s writer Roald Dahl. It starts off breezily, heading into what seems like drawing room drama, before becoming as dark and sharp-toothed as one of Dahl’s fictive monsters.

It is 1983, Dahl (John Lithgow, fabulous, and bearing uncanny resemblance to the writer) is just about to publish The Witches. We find him irascible, in a kingly, upper middle-class way, having just moved into a new home while his publisher, Tom Maschler (Elliot Levey, excellent as ever) and soon-to-be second wife, Felicity (Rachael Stirling) buzz around him in an unfurnished kitchen.

The drama revolves around an explosive book review that Dahl has written, railing against Israel’s invasion of Lebanon in 1982. We hear how he has spoken passionately about Palestinian oppression in the past, and now is writing against Israel’s wholesale killing in Lebanon – in language that some deem antisemitic.

This kitchen gathering is something of an emergency meeting. “We can make it go away,” says Maschler, a survivor of the Holocaust who has little allegiance to Israel and great loyalty towards this writer-friend.

The central conflict is triggered when an American Jewish sales executive enters the room. Jessie Stone (Romola Garai, restrained but ready to burst) has been sent by Dahl’s American publisher as a damage limitation exercise. The plan is to get him to apologise, but everyone creeps around this star author, not wanting to upset him, at first. He delights in his power, referring to Jessie’s Jewishness in provoking ways, and we feel the temperature drop when she begins to bite back.

She accuses him of conflating Israel with Jewishness, and challenges his comparison of Israel to Nazi Germany. He speaks of apartheid, of the systematic degradation of Palestinian life and the responsibility of Israeli citizens to speak up in protest.

It is sophisticated writing, speaking not only of Dahl but also to our own time, although the ground is inherently lopsided: the opposing arguments around Israeli and Palestinian freedom cannot be weighted equally when one – Dahl’s – is fuelled not only by a sense of righteous injustice but also bigotry.

He is no straightforward monster, though, or at least not in the first act, when he is also rational, tender and playful. Rosenblatt’s writing steers delicately away from polemic or crude binaries. Dahl speaks of “your lot” to Maschler and generalises about Jews as a “race of people” bearing certain traits, alongside legitimate criticisms of Israel.

By the second act, his antisemitism is glaring, and the drama seems to not know where to go from here, stalled by having to return from the coded conversations of our day back to the fall-out around Dahl’s article.

Until then, so many debates are embroidered seamlessly into the drama, from the gap between the monstrous genius and his work (Stone admits she still loves Dahl’s books), to the exploration of Jewishness. (Maschler, as a Jew, never defines Dahl as an antisemite).

Where some theatres have remained at a safe distance from this subject matter – the Royal Exchange theatre in Manchester has recently been accused of censorship on it, for one – Giant shows a necessary bravery in taking it on. This is what theatre is for.

• At the Royal Court until 16 November