When Richard O'Brien's musical The Rocky Horror Show was first staged late at night, at the small Theatre Upstairs at the Royal Court in 1973, it rapidly became a cult phenomenon.

The B-movie-inspired story of the naive Brad and Janet entering into a world of transvestites, aliens and fishnet stockings went on to become a Broadway production.

Then in 1975, the film adaptation The Rocky Horror Picture Show came out, which according to reports, has the longest theatrical release in cinematic history, and can still be seen on the big screen today.

With fans crying out for more, O'Brien then penned a sequel Shock Treatment, which was released as a film in 1981 and featured many of the same actors and songs of a similar ilk.

However, the film which follows Brad and Janet into a town that doubles as a TV station, failed to get a theatrical release and remains little known even to some Rocky Horror fans.

Now, 34 years later, Shock Treatment is going back to Rocky Horror's roots and is being staged as a play for the very first time at the King's Head Theatre in London with the blessing of writer Richard O'Brien and music composer Richard Hartley.

Hartley, who turned out for the show's dress rehearsal, said: "The film was a bit of a mess really but we always thought we had some good songs and when Benji approached us about putting it on stage, I thought why not? It's been completely reimagined by Tom Crowley and it's become a show for today."

Benji Sperring, the director of the show, said: "I think the biggest shock is in the original movie there isn't any shock treatment. They don't really explore that so we've put it in," he said, adding: "It's going to be really different. If they know the original film, they'll see where it comes from and they'll see what the starting block is but the path that we've taken to adapt it for the stage is really different."

Sperring pursued O'Brien for nearly 10 years to get the rights to stage the show, and finally the actor and writer relented but with one specific instruction – to have the production put on in a small venue like Rocky Horror.

"When I was talking to Richard Hartley and Richard O'Brien, they wanted something which was up close, that there was no escaping, so we really focused on that, alongside making sure that we were able to explore all of the avenues that were needed, that we could go through the audience, that we could go into the audience, that we could engage with the audience and bring them up on to stage," said Sperring.

Both the play and the film of Rocky Horror have become interactive experiences, with audiences dressing up and interacting with the characters on screen or on stage. Sperring has taken into account that the same thing could happen with Shock Treatment.

"We've got no idea if the audience will get up and get involved or if they're going to sit back and be passive or if it's going to be a very active piece. We don't know which way it's going to go," said Sperring. "The actors are prepped as much as they can be and they're aware that they're just having fun. Richard O'Brien said very early on: 'Let fun be your guide and as long as you're having fun, the show will be all right.'"

The show features West End performer Julie Atherton as Janet, and ex-soap star Mark Little as Farley Flavors, the media magnate who owns the TV-show town Denton.

Shock Treatment is running until 6 June at the King's Head Theatre in Islington, London.