Now politics and showbusiness are locked in a mutually adoring embrace, there could be no better moment to present Peter Morgan's enthralling semi-documentary drama about the worst political scandal of the American 20th century.

Frost/Nixon captures the excitement of trying to make a historic TV programme and the struggle to discover how to beat a politician at his own game.

The play shows the process by which, three years after his enforced resignation, President Nixon is coaxed onto a fourhour TV interview. Nixon's aim is to revive his ruined reputation by submitting to questions from a young television chatshow host, that oleaginous admirer of stars in their courses, David Frost.

In the highly theatrical event we watch how Nixon steered a comically evasive, sentimental, artful course through the soft barrage of not very skilfully put questions from Michael Sheen's slightly camp and politically vacant Frost.

The final programme, though, thanks to a discovery by Frost's researchmanthe famous anti-Nixon writer James Reston, changes everything. A killer question just about slaughters the president.

Frank Langella's amazing performance, one which nobody interested in great acting should miss, catches and registers those final moments when Nixon is left disorientated, confused and incoherent. In Michael Grandage's suitably hectic, excitingproduction a huge TV screen magnifies the interview scene that is enacted beneath it.

As a shifting, shifty Nixon, Langella's face takes on a weird, new life of its own. It seems to grow grosser, swollen and contorted by stress.The jowls and eyes droop. He licks his lips again and again as if to rid them of some bad taste. His eyes widen, his voice thickens and fades out. He admits to letting down his country and involvement in the attempted cover-up of burglary and bugging of the Democrats' offices at the Watergate hotel. Nixon had been destroyed by television with a thoroughness politicians could not manage.

Langella, who exudes authority as naturally as if born with it, has more than something of the man's throaty, throttled voice, his strange vulnerability and cunning. He makes the final fall of this wicked president as abject, pathetic and sudden as Icarus's or as awful as Lucifer's: life drains out of him like air from a tyre.

It may sound like a will-he-admit-he-dunnit or a trial by television drama, for Morgan reflects the edgy, financially insecure world of television programmemaking: a young John Birt hovers uselessly around and Elliot Cowan's impressive James Reston serves as an angry narrator. Interestingly Morgan characterises Nixon and his entourage as borderline odd: Corey Johnson's Chief of Staff and Nixon himself spend time brooding over Frost's laceless Italian shoes and classify them as effeminate. Yet the play is far more intriguing and psychologically nuanced than that.

For Morgan adds to the documentary material. His provocative notion is that Nixon saw Frost as the sociable, clubbable sexual success he would have liked to have been. The president displays a strange kinship with him too.

In a drunken late-night call he assures a sceptical Frost they are both humble outsiders in a high society of intellectuals and achievers, that they are both struggling to make a comeback through a TV battle which only one can win. Frost/Nixon, in a fashion at once chilling, memorable and comic, warns us that politicians now live and die by television.