The Mousetrap - by a whisker

Shortly before 9am tomorrow (NZ Time) - all things being equal - a large black car will pull up outside a small theatre in West Street, midway between Leicester Square and Covent Garden in London's West End.

The Queen, perhaps relishing the prospect of a couple of hours' distraction from her recent preoccupations, will alight and climb the few steps into the foyer to preside over a remarkable anniversary: The Mousetrap, the longest-running play in history, is 50.

As a milestone, extraordinary though it is, the play's 50th birthday has a significance which is numerical rather than record-breaking: it became the longest-running production in British history on April 12, 1958, when it passed the five-year record of Chu Chin Chow and the longest-running in the world more than a quarter-century ago in the mid-70s.

These days The Mousetrap is the sum of its numbers. The promoters say "more than 10 million" have seen it, but only those who have never been to any other play would be likely to regard it as a good, or even interesting, piece of theatre. It long ago became much more - and much less - than that: a record-breaking curiosity, a fixture on the tourist circuit and a legendary part of theatrical folklore.

Even before you enter St Martin's Theatre, everything about The Mousetrap proclaims its determination to be assessed not for what it is, but for how long it has been it.

The pink neon sign, half the size of the theatre frontage, has the name of the author - one Agatha Christie - in smaller letters than the words recording the length of the show's run. In the small foyer, the crimson carpet is threadbare in places and a scoreboard-style counter (which tonight will read 20,807) marks the number of performances.

"This isn't the theatre, dammit!" everything seems to say. "Why, it ain't even show business. This is History."

In this pageant, the front-of-house staff play their parts with a faintly distracted air, like old troupers working from an ancient script; a small souvenir stand sells sweatshirts and mugs and - oddest of all - Mousetrap mousepads, 21st-century merchandise which seems out of place for a play written when the word "mouse" had only one meaning.

Meanwhile the usher who shows you to your seat to watch this most English of plays is less likely to be English than an Australian on her OE, perhaps, or a young Spaniard improving his English.

On stage, projecting into a theatre which, on this Tuesday matinee at the height of the tourist season, is barely one-third full, the actors do their best to breathe life into an old-fashioned whodunnit.

The story, like the set decor, is pure Christie: Detective Sergeant Trotter arrives at snowbound Monkswell Manor in Berkshire with grim news. A murder has occurred in London and he has reason to suspect another is imminent - in the manor, which is crowded with sudden arrivals. Depending on the angle of view, each of the seven guests seems suspect.

This is theatre from the age of the footlight. The lines are declaimed by a cast which is often arranged laterally, like birds on roosts, across the stage. Actors virtually wink if called on to deliver a line with a clue embedded in it. And the audience plays along with the vaguely camp farce: we brace ourselves for the gunshot - yet when it arrives, we giggle rather than flinch.

The denouement involves a hat box from Harrods, but the resonances set up by the name of the Knightsbridge store owned by Mohammed al Fayed make the reference odd and awkward.

By the time the cast joins hands for a rather offhand bow and we are enjoined as "members of The Mousetrap family" to keep its secrets, it feels faintly like being caught in a time-warp.

That's hardly surprising. It's an entertainment, after all, of the 1940s, which was born when Queen Mary asked the BBC for "a new Agatha Christie" (the prolific author was a royal favourite) to mark her 80th birthday.

Those were the days when monarchs spake and subjects curtseyed. The writer obliged with a 30-minute radio play called Three Blind Mice.

Nothing more might have been heard of it, but Christie, who called herself "a perfect sausage machine", decided to add extra characters and created a stage play which runs for almost exactly two hours.

The Mousetrap opened in Nottingham in October 1952, and briefly toured the Midlands and the north of England before settling into the Ambassador Theatre in the West End, 50 years ago today. It transferred to the larger St Martin's Theatre next door on March 25, 1974.

No one, least of all Christie, imagined such success. Five of her plays and another based on a novel had been staged. Some had flopped, though the most recent, The Hollow, had enjoyed a run of 376 performances.

But Christie doubted The Mousetrap would last more than six months and her instinct was not far wrong: for two years, with husband and wife Richard Attenborough and Sheila Sim in the main roles, the show prospered. But when they left in 1955, the box office plummeted. The theatre even gave the production notice to quit.

What happened next has been called the Mousetrap Effect, though it's a phenomenon familiar to any showbiz types who have seen the way a buzz can rescue a dying show or turn a slight success into a blockbuster: news of the imminent closure boosted ticket sales as people crowded to catch a performance. Success built on failure and then on success.

"Now," director David Turner told the Guardian last month, "if we put a notice up saying 'closing month', we'd probably be busy for two decades."

The rest, as they say, is history. The Mousetrap has become a sort of theatrical work of perpetual motion, and the reason for its longevity is anyone's guess.

Christie herself would remark years later that the play had "a bit of something in it for almost everybody" (though it's hard to see what it has in it for 21st-century audiences).

It has become a cultural paradox, an ageless anachronism. Why it should have so outperformed, for example, Witness for the Prosecution - which opened a year later and ran 468 performances in London before moving to New York for an even longer run - remains the play's real mystery.

No one could be happier about the play's success than the writer's only grandson, Mathew Prichard, who was 9 when it was written. When Christie handed a package of pages to producer Peter Saunders in early 1952 with the words, "This is a little present for you", she had already arranged to leave the play in trust for the boy.

It turned out to be young Mathew's lucky day: the West End takings are well over $100 million, never mind the royalties from other productions. As an act of generosity, if not philanthropy, it rivals J.M. Barrie's gift of Peter Pan to the Great Ormond Street Hospital for Sick Children (the hospital still banks the Disney cheques from time to time) and Handel's donation of his Messiah to a hospital for foundlings.

Spare a thought, by contrast, for John Woolf, a producer who bought the film rights in the mid-1950s. A clause in the contract provided that no film may be released for six months after the play's final performance - a restriction designed to protect the play's advantage of surprise about who actually did dunnit.

Woolf was plainly no slouch at picking a winner - his credits include The African Queen, The Day of the Jackal - but he died in 1999 without the chance to exercise his option.

His mission has been picked up by his son, Jonathan, who has reportedly commissioned a darkly brilliant script. But the owners of the copyright are refusing to relax their contractual hold. The lawyers are exchanging letters.

In the meantime, the play's secret will stay within the walls of St Martin's for as long as people keep paying to hear it. The curtain rises tomorrow on the second half-century.

Snap Shot:

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* When The Mousetrap opened, Winston Churchill was the British Prime Minister, Harry Truman was US President and Stalin ruled the Soviet Union. Britain was still in the grip of postwar rationing.

* The producers say "over 10 million" people have seen The Mousetrap, although that figure may be overstated since it implies about 95 per cent houses, although long, flat box-office periods have often been remarked on.

* Tomorrow's 50th-anniversary performance is number 20,807 - the play has been performed in 24 languages in 44 countries.

* Takings in the West End are more than $100 million.

* A total of 332 actors have appeared with 167 understudies under 20 directors.

* Apart from credit as the longest-running theatrical show in history, The Mousetrap has two other entries in the Guinness Book of World Records: David Raven's 4575 performances between 1957 and 1968 as Major Metcalfe make up the longest single engagement by any actor in one part; and Nancy Seabrooke understudied the role of Mrs Boyle for a record 6240 performances over 15 years - appearing on stage only 72 times.

* The Mousetrap's longest-serving wardrobe mistress, Miss Maisie Wilmer-Brown, has ironed almost 70km of the more than 160km of shirts worn in the show.

* The revolver from the original production of The Mousetrap was auctioned in 1985 and is now part of the Victoria & Albert Museum's Theatre Department.

* One item still remains from the original set of 1952: the clock on the mantelpiece.

* With worldwide sales of two billion in more than 50 languages, Christie is the best-selling author in history (the Bible, Mao Zedong's Little Red Schoolbook and Shakespeare's works have sold more, but many of those purchases were institutional or mandatory). When she died in 1976 she had 66 novels, 14 collections of stories and 21 plays (including 13 full-length stage plays) to her credit.