Animal magic: playwright Morna Pearson on her zoo-based play, Dr Stirlingshire's Discovery

THEY have moved us deeply with a drama performed in a children's play park (Decky Does A Bronco), weaved a romantic thread through the rooms of a semi-dilapidated Edinburgh townhouse (Those Eyes, That Mouth) and taken us on a political and psychological journey beyond the security cordon of Edinburgh Airport (Roam). Now, in co-production with Lung Ha's Theatre Company and the Royal Lyceum Theatre, Edinburgh, site-specific theatre company Grid Iron are offering us an exciting chance to experience a work performed in promenade around Edinburgh Zoo.

Written by Elgin-born playwright Morna Pearson and entitled Dr Stirlingshire's Discovery, the drama is part of the Edinburgh International Science Festival programme. The play is set, Pearson tells me when I meet her in Edinburgh, "in a very heightened, fictional version of Edinburgh Zoo".

This reimagined zoo is run by a manager, Henry Stirlingshire, whose professional ethics are slightly dubious. None-too-fond of his sister, Dr Vivien Stirlingshire, a "cryptozoologist" who scours the planet seeking undiscovered animals, Henry arranges a grand unveiling of Dr Vivien's latest find.

Henry and Vivien have "never got on", says Pearson. "So, it's kind of a tale of sibling rivalry."

In fact, Henry doesn't believe that Vivien has discovered a new animal at all. "He thinks his sister just swans around the world making stuff up," the playwright explains. "He thinks he's setting her up for an embarrassing fall."

Has Dr Stirlingshire discovered some amazing creature, an equivalent of the Yeti or the Loch Ness Monster, perhaps? Or, will her "discovery" turn out to be a humiliating dud?

"There is a secret," says Pearson. "We want it to be exciting."

A play presented within the Zoo in the evening, after it has closed to the public, is bound to generate a frisson of excitement. Involving a carefully planned evening stroll through Scotland's premier zoological institution, the piece has presented the team with practical challenges which have amazed the playwright.

Pearson is impressed by the logistical work involved in turning her play into a promenade performance around one of Scotland's favourite visitor attractions. "I thought the Zoo after hours would give us more freedom. But that turns out not to be the case, because some animals have to go to bed," she says with a laugh.

She is glad that Grid Iron are so accustomed to the challenges of site-specific and promenade performance. "Thank goodness," she says, "because some of the problems that came up blew my mind."

Pearson is, like most playwrights, a writer for the stage. Creating a promenade piece to be performed in a major zoological gardens was a very new and interesting experience for her.

"Sometimes you just want to get on with the story, but you can't," she admits, "because the route won't let you. Maybe, at a certain point, you'll have to go round the long way, and you have to think about what to put along the route to entertain people while they make the journey."

She has, she says leant heavily on the expertise of Grid Iron and Lung Ha's in fitting her writing to the very specific circumstances in which the play is being presented.

Creating a play for performance in the Zoo has required a different style of writing, she says. Her characters, who will be speaking to a crowd of people, often outdoors, have to be "more shouty" than usual, as they make proclamations, a little like a town crier of olden times.

Many of those difficulties in creating the show relate to matters of accessibility, both of the play itself and the route it follows through the Zoo. While Dr Stirlingshire's Discovery is not being described as a children's show, it will, says Pearson, "work better if there are children in the audience".

The piece is being presented as a "relaxed performance", she continues. "It doesn't matter if the children make a noise or chat."

It is particularly important for the show's co-producers that it be accessible to children and adults with learning disabilities. Lung Ha's is, after all, Scotland's acclaimed theatre company for people with disabilities.

People with learning disabilities, their loved ones and carers are assured that both show and route will be accessible to disabled audiences.

It is a curious coincidence that Pearson's show should come just weeks after a major controversy involving the South Lakes Safari Zoo in Dalton, Cumbria. That zoo, in which close to 500 animals died in four years and a 24-year-old zoo keeper was killed by a tiger, has faced calls for closure.

I wonder if Pearson shares any of the wider ethical concerns that many people have about zoos in the 21st century. "There's a case for zoos that prioritise conservation and welcome questioning, both of which Edinburgh Zoo do," she says. "It seems to be a zoo that keeps up with the times."

Whatever one's opinions on that vexed issue, it's clear that Dr Stirlingshire's Discovery is a tantalising prospect. Offering a cast of more than 20 actors and a journey into the mystery of Pearson's play, it will appeal, its author hopes, to "people who go to the zoo, but don't go to the theatre, and people who go to the theatre, but don't go to the zoo".

Dr Stirlingshire's Discovery is at Edinburgh Zoo, April 1-9: gridiron.org.uk