The plot, such as it is, involves corsets, cannibalism and a mad transvestite scientist from Transylvania, who turns out to be an alien transported to Middle America, while the songs are camped-up Sixties soundalikes. So it was always going to be a big hit. But even its creator Richard O'Brien could not have predicted, when he wrote The Rocky Horror Show more than 30 years ago that it would have become the cultural and social phenomenon it is today, inspiring such devotion among its many fans.

Tomorrow night they will be out in force at a special tribute performance of The Rocky Horror Show at the Royal Court Theatre in Sloane Square, where the show was first performed. The evening, in aid of Amnesty International, is a centrepiece of the 50th anniversary celebrations of the Royal Court, and the Rocky Horror experience was recently voted the most enjoyed piece at the Royal Court during its 50 years.

Those taking part include several members of the original cast - O'Brien himself and others, including Anthony Head and Adrian Edmondson, who have appeared in its various revivals. Tickets have been changing hands for as much as £800.

But as O'Brien said yesterday: "It was a hot ticket from day one." First staged on 16 June, 1973 in the Royal Court's upstairs room, with a capacity of less than 100, the RHS wowed critics from the start who voted it best new musical of the year.

It was a life-changing moment for O'Brien, who was a jobbing actor in his late twenties, just sacked from Jesus Christ Superstar after one performance, and thinking about packing in his acting career for good.

He took his inspiration from science fiction and horror "B" films. "I'd sit there late at night and watch these films on television like They Came from Outer Space and The Day The Earth Stood Still as well as the old Boris Karloff movies, which were before they discovered the idea of comedy in horror films."

On top of that came gender play, embodied by the cross-dressing main character of Frank-N-Furter. "This was at a time when the whole gay community was coming out of the closet and when the glam rock thing was very big." David Bowie's pop-star-meets-space-cowboy creation Ziggy Stardust was huge at the time O'Brien was creating the RHS.

O'Brien admits that it served as an outlet for his own personal sexual journey. "I wouldn't say I was gay, I'm in between, on that spectrum that runs from man to woman, as a great many people are. It was an outlet for my own feelings.

He added: "I didn't really understand what I had done until I saw Tim Curry up there in his corset and fishnets singing "I'm A Sweet Transvestite". But then, all of the characters had a bit of me in them."

Now it seems tame, but at the time fetish wear, cross-dressing and homosexuality on stage was groundbreaking. The show made a star of Curry, who electrified audiences as Frank-N-Furter, the cross-dressing mad scientist who entraps an innocent young couple, Brad and Janet.

The rest of the story revolves around Rocky, a man who was built by Frank-N-Furter for his own, er, pleasure; the latter's servants, Riff Raff the demented butler and Magenta; a groupie, Columbia; and a visiting biker, Eddie.

During the course of proceedings, Frank-N-Furter deflowers both Brad and Janet, kills Eddie, serves him up for dinner and hypnotises most of the cast to perform a floor show dressed in corsets.

O'Brien played Riff Raff; Julie Covington, who would later star in Evita and Rock Follies, played Janet; the Australian actress Little Nell, also known as Nell Campbell, played Columbia; and Patricia Quinn played Magenta.

After a short period in Sloane Square, it transferred to a much larger theatre on the King's Road in Chelsea, where it ran for six years, followed by another 18 months in the West End proper, closing in 1980 after almost 3,000 performances.

By this time, it had been filmed as The Rocky Horror Picture Show with Curry, Campbell and Quinn, and also featuring Meat Loaf and Susan Sarandon.

And the show goes on. There have been provincial tours and a major revival in London in 1990 with Adrian Edmondson and Tim McInnerney in the cast. There have been productions in different parts of the world - one in New Zealand featured a young Russell Crowe playing Eddie - and it has been translated into 10 languages.

And now the show is touring the UK with Suzanne Shaw (from Hear'Say) and David Bedella (who played the devil in Jerry Springer the Opera) as Frank-N-Furter.

What has taken RHS into another level of showbusiness is the extent to which it involves the audience. It started in the United States, where the film, never a huge hit, attracted a small number of fans who would go and see it time and time again, dressing as the characters.

Then came the point where audiences began to join in, which is known as "talkback". There is a scene when Brad and Janet knock on the door of the castle, which is opened by Riff Raff. In the pause before the character spoke, one audience member at a screening in the United States is said to have shouted out: "What your favourite Lionel Ritchie song?" The actor said "Hello..." which is, of course, a Lionel Ritchie song.

According to Stephanie Freeman, who runs the show's British fan website, a tradition was born, one which varies depending on on which side of the Atlantic you live. In the United States, fans follow the film, which is being shown somewhere almost every week, dressing in precise reproductions of the costumes, while the talkback relates to more local matters, such as hamburger chains unknown in Britain. Over here, things are more anarchic. "It varies according to where you are and what it going on," Mrs Freeman said. "Some of the talkback can be very up to date, referring to whatever's on the news that day." For instance, in response to the line, "It's not easy having a good time," the audience invariably respond "It is in..." shouting the name of whereever the show happens to be on. It is also obligatory to shout "arsehole" every time the name of one character, Brad, is mentioned.

Other rituals in which the audience is expected to participate include throwing rice, during two wedding scenes, and toast during the dinner scene, firing water pistols when it rains (at which point it is appropriate to put a newspaper over your head) and shining torches during the song "There's a Light". It is also important to take along a pair of rubber gloves, "to be snapped in time to Frank-N-Furter during the creation scene," according to the "Virgins' Guide" section of the Timewarp website run by Mrs Freeman (named after the show's most popular song).

The show has joined a group of musicals, including Abba and The Sound of Music, which are now staged in special "singalong performances" where the audiences join in, karaoke-style.

British costumes are more free-form than in the US: "People wear corsets and lab coats but also just dress wildly in whatever they want. The whole point is to go against convention, have a bit of a laugh, be a bit risque," said Mrs Freeman.

An office worker from Kent in her early forties, she normally wears a variation on Magenta's original French-maid outfit but is currently donning the net frock of the present stage production.

Her husband, David, a university photographic technician, usually kits up as Riff Raff, with tailcoat, stringy wig and spattered blood; they have 19 different costumes in their loft. The couple met in 1988 through their enthusiasm for the show, which they have seen hundreds of times. They have been married 16 years and have two young children.

She laughingly rejects the idea that you have to be an exhibitionist transvestite to enjoy the experience: "We are a perfectly normal couple. Although you do get a few transvestites, it's mostly about having a good night out."

For them, the show signals freedom: "It's about going out and having a good time, having the freedom to be what you want to be. You get to meet people. For some it is a right of passage and for others it's a much more long term thing."

For Patricia Quinn, (widow of the actor Robert Stephens) the role of Magenta has been "my Moby Dick". "I feel like Jonah, tied to the whale." As well as the original show and film, she has appeared in tours, one-offs and revivals, appears at fan conventions and will be there on Wednesday night. "It's extraordinary, it doesn't get less, it gets more. I love it all."

For Richard O'Brien, the RHS has been a lifetime's work. "It's earned me a decent living over the years, but it hasn't made me a rich man. That's rather a good thing - I wouldn't want to have ended up like Andrew Lloyd Webber, would I?"