Had Michael Morpurgo put “When” in the title of his children's book - which became a film with Paul Scofield and Helen Mirren and is now a play - we could have expected a jolly adventure, honouring the wonders of nature. But a title that begins with “Why” at once introduces a mystery, and those whales, when at last they appear, will also have to answer that other question, posed in the first moments of the play: “What is the Curse of Samson Island?”

Samson is little more than an islet, lying a mile south of Bryher, one of the larger Isles of Scilly, and it is mostly on Bryher that Morpurgo sets his tale, in the years before the First World War. Samson used to be inhabited but about 50 years previously some catastrophe drove the survivors, a mother and son, to take refuge on Bryher, where they have been feared and shunned ever since. The mother died long ago and her son is a reclusive fisherman who shouts at birds and is known as the Birdman.

Gracie, a young schoolgirl, and her friend, Daniel, share the general terror of him but establish a tentative friendship that leads to an understanding of the tragedy and why the whales once came and now have come again.

The adaptation by Greg Banks, who also directs, makes Gracie the narrator of the story, and Eliza Caitlin Parkes moves smoothly between those moments when she takes the audience into her confidence and those when she plays the plucky child who can't keep the fear from her voice. She and Jay Quinn's Daniel at first communicate with the Birdman by leaving messages in shells upon the sand, and on the Old Rep Theatre's deep stage Jacqueline Trousdale's design creates a lively suggestion of beach and fishing hut, dominated by a red sail and broken mast. When Banks needs to show the two children adrift at sea he has them rocking on a plank balanced on a crate. It's simple and brilliant.

Best of all is the rescue of a stranded narwhal, represented simply by its 10-foot long tusk (in days of yore supposed to be the unicorn's horn). Led by Chris Llewellyn's hoarse-voiced Birdman the cast of six heave again and again at an invisible shape, and the theatre itself seems to give a gasp of relief as the creature at last reaches deep water.

Llewellyn's air of desperate sorrow haunts a production, well cast throughout, that combines a reverence for nature's wonders with mystery, excitement and passion.

Box office: 0121 303 2323, to Jan 24

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