Nora is desperate. She snatches up the phone crying "Operator, get me Sing-Sing, and hurry!". Hey, Lady, what's the rush? Well, everyone thinks Nora's locked up in a cell for murdering merchandising magnate Spencer Wylie. But she's not. The woman in the cell is none other than her sister Laura... and she's frying tonight.

You may not believe this, but although The Betrayal of Nora Blake is the first musical to feature the enraged threat "You'll never work in a New York correctional institution again!", it's not the first to feature the electric chair. That honour goes (I think) to Sondheim's Assassins. The chair was back in the ill-fated but fabulously dreadful The Fields of Ambrosia... ("Where everyone knows ya") but that show not only failed to answer the accusation of bad taste, it never even understood the question, making it a must for all theatrical ambulance-chasers. But that pales into insignificance beside the black and white glories of The Betrayal of Nora Blake.

Its credentials are impeccable. Billed as "a musical noir", it takes every great late-night B-movie you've ever seen, puts them in the blender with torrid Hollywood film scores and comes up smelling of fishnet and chickenbones. Pardon?

The plot revolves around two designing women, man-eating Laura and "sweet, trusting, naive little Nora" who just happen to be identical twin sisters - heck, even their wigs are the same. It's 1945 and, as the song says, "Uncle Sam is using all the crepe-de-chine". But the sisters decide they're going "to haffeta do without taffeta" and hit the big time with their impossibly glamorous first collection, turning fishnet and chicken bones into a fashion sensation. Cue the ridiculously funny all-singing, all- dancing fashion parade production number with Christopher Woods's eye- popping frocks and murderous millinery. Philip Treacy, eat your heart out.

Woods's set and Nick Richings' lighting work wonders, taking wit and sincerity as their guides. It is those two qualities which lift this from sweet nonsense to superbly executed high camp. The book, music and lyrics by John Meyer (where has he been all this time?) may be a small-scale giant parody, but the detail is superb. The music harks back to the wailing saxophone from A Place in the Sun, the slinky, swoopy theremin sounds from Spellbound and, most of all, David Raksin's tritone-filled, minor-key love theme from Laura.

But this is not just some cut-and-paste job. Meyer's lyrics have gloriously silly, neat rhymes and his pilfering of the past has comic and dramatic purpose.

Best of all is Nickolas Grace's direction. He encourages his excellent cast to mine the text for character, situation and, yes, truth. The result is just deliciously funny. Claire Moore is tremendous as Nora, looking like a wide-eyed, innocent Joan Fontaine and sounding exactly like Joan Crawford at her breathiest best. She's got comic timing in spades, whether she's hilariously saving Snorkel the dog or escaping the clutches of Jeffrey (Michael Matus), who has bad thoughts and worse breath and provides the funniest French-kissing on the London stage. Issy van Randwyck turns in another helium-on-legs performance as double-crossing Laura while John Levitt comes on strong as the analyst "viz ze truth drug" in the shape of an outlandish hypodermic.

The run has already been extended and anyone with money and half a brain should ditch the poster and take it into a small West End theatre. But in case this dream never comes true, spoil yourself and book immediately.

Jermyn Street Theatre, SW1 (0171-287 2875) to 7 Jul