Beyond the Horizon / Spring Storm, National Theatre: Cottesloe, London

Beautifully directed by Laurie Sansom, these paired productions that began life at the Royal & Derngate Theatre in Northampton constitute a triumph on so many levels that I would need at least twice the space to do them justice. With cuts in the arts looming, they represent a supreme vindication of the principle that regional theatres and the National should be – in a way only possible through enlightened subsidy – organically related. It is a feather in the cap of Nick Hytner that he has given these productions the showcase they deserve.

The primary laurels go, though, to Sansom, who spotted the unsung brilliance of these two plays. Beyond the Horizon (1920) is Eugene O'Neill's first full-length play and marks the dawn of American tragedy. Spring Storm (1937) is the product of a playwriting course undertaken by Tennessee Williams in his mid-twenties and came to light only after his death. Samson divined the extraordinary affinities between these two marvellous works and honoured them in the casting, design and ethos of these splendidly acted productions.

Though very different in tone, both plays offer clear yet tantalising glimpses of what these playwrights would realise later. In Beyond the Horizon, two farmer brothers fall for the same girl but through a ghastly quirk of misprision, the bookish dreamer (lovely Michael Malarkey) stays behind on the land, where he fails as farmer and as husband to his mountingly discontented wife. In a performance of quite stunning insight and theatrical power, Liz White bitterly and belatedly recognises that her heart was with the brother who went off to sea and then wealth on an estate in Buenos Aires. This brother is portrayed in all his endearingly clumsy guilelessness by the excellent Michael Thomson. The dreadful but utterly persuasive and completely melodramatic twists that propel all three to an intricately mutual despair put the audience through the mangle. And you would not want to be anywhere else.

White, with a consummately contrastive portrayal of Heavenly Critchfield, the young Blanche Dubois-in-waiting in Spring Storm, announces the birth of a new star and a formidably dazzling actress.

To 22 July (0207 452 3000; Nationaltheatre.org.uk)