How do the grubby little creatures do it? Joe Penhall's new play, Dumb Show, poses the question of how those human bluebottles from the tabloid press, who swarm around the houses of celebrities attracted by the stench of scandal, face up to the mirror each morning.

The play presents two journalists (if that word can encompass both the dirty raincoat brigade and Bernard Levin) working undercover to entrap a television comedian who has problems with drink, drugs and his marriage. The victim, Barry, is seedy, pompous and a sucker for flattery. OK, he is trying to stay off the booze, and we don't know whether he would make a pass at a pretty girl if she were not so obviously giving him the come-on - but still, we are given little reason to respect him. Yet, however manifest his shortcomings, my sympathy (and I think the audience's) is with him as his tormentors spout all their self-deceiving nonsense about their duty to lay bare his sins, and the invaluable service to the public interest that their day's work will represent.

Penhall has an awesome ear for dialogue. Early in the play, a bank manager and his secretary tout for the celebrity's custom. The playwright captures their patter perfectly, including the secretary's stupid repetitions of her boss's cliches. There is a coarseness in their exchanges that suggests there's hanky-panky going on between them. With their ill-disguised petulance towards each other, they embarrass themselves and their prospective client. She tries to laugh off the man's crass innuendo with: "Sorry. Office politics." It sounds wonderfully authentic.

Penhall's sureness of touch is also evident when the two bankers lavish flattery on the media star. They outbid each other in their praises for every one of his banal utterances: "Brilliant, deep, very deep, the insight, very wise, astute." There is almost no limit to the idiocies that people will trot out to a face they have seen on television, especially if they are hungry for business.

Anna Maxwell Martin as the muckraking temptress, Liz, shows herself off in a black cocktail dress that might cause a neutered saint to forget his vow of chastity. She extols her flute of vintage champagne in those flat vowels that suggest a girl of humble origins who has shinned up the sleazy pole by ruthlessly deploying her body, cunning and ambition. She can effortlessly turn on naivety, compassion or flirtation as the moment demands. When turned off, they leave no trace. She has a nose for a story and a cold-bloodedness that make the male of the investigative species look like a sentimental patsy.

Rupert Graves is the male, the senior scumbag Greg, whose mind is so contorted with envy and hatred that he has come to love his sordid work. "Fame is a cancer . . . a plague . . . it's our responsibility to take him down a peg or two . . . Because if we don't, nobody else will." Greg is truly immoral, inebriated with resentment and high on journalistic sadism. He lives a life of putrefying corruptness that eclipses Barry's peccadilloes, but you could never sell newspapers with an expose of red-top ethics.

Barry's fragile mental state is nicely portrayed by Douglas Hodge. He pours out his contempt for his television audience ("lobotomised wankers") in a diatribe that merely betrays his self-loathing. He needs to be the centre of attention, and characteristically sets about seducing the girl with a torrent of confessions designed to emphasise his vulnerability and draw her sympathy. He cuts a pathetic figure as he descends into drunkenness, but is briefly magnificent when eventually he emerges from the wreckage of his life.

Dumb Show succeeds in revealing another truth: that the victim can be humiliated, flattered or bribed into conniving with his destroyers. The comedian cannot resist the microphone, even the one that records his self-indictment in words pre-packaged for tabloid publication ("I blame the fame game"). The public figure cannot quite resist the perverse lure of being exposed - which is fame of a sort - the opportunity to experience, almost enjoy, 15 minutes of notoriety across the Sunday breakfast tables of Britain.

Following on from Penhall's success with Blue/Orange, which won three best play awards and transferred to the West End and Broadway, it seems likely that Dumb Show will have a life long after its debut at the Royal Court, and deservedly so.

At one point, the journalists think they have discovered that Barry's wife is very ill. They call her up, but she tells them they have got it wrong. Her denial puzzles them. Barry shrieks at them that she has lied to them simply because her state of health is private. That seems to be a word the pair of scribblers - I mean, brave soldiers in the struggle for truth and the public interest - cannot grasp. Barry goes on: "You have zero imagination and so humanity is a mystery to you . . . the stink of your own spleen and bile - the pain that you inflict - is a mystery to you."

Good stuff, Penhall.

Booking on 020 7565 5000 until 16 October