Why the 23-year-old Mozart wrote two acts of an opera in 1779 and then abandoned it remains a mystery. He may have heard rumours of the planned German-language company in Vienna that would later premiere Il Seraglio, whose narrative also deals with slavery and freedom in an Ottoman empire context. But this substantial fragment has proved tempting over the years to Mozart specialists, who have tried to complete it in various ways. The latest attempt is this English-language version by conductor Ian Page, working with writers Michael Symmons Roberts and Ben Power, and director Melly Still.

The fact that there is no third act and the original spoken dialogue has also gone missing means they have their work cut out even to establish a coherent plot. In the event, this is scarcely achieved. The revamped scenario, set in an unidentified contemporary prison camp, is too general to be involving and too obscure in motivation to evoke sympathy for characters who are nevertheless abused in diverse, mostly sexual, ways. Then there is the tacked-on third act suddenly that provides an incongruously happy resolution.

It does not help that the dialogue is poorly delivered, but even with perfect timing and enunciation, one spoken scene in the second act is far too long for its context. Page's decision to end the show with a familiar quartet from Il Seraglio also highlights the fact that much of Zaide is immature Mozart.

Pumeza Matshikiza sings the title role appealingly, but only Amy Freston's Perseda and Simon Lobelson's Osmin manage both speech and song with success – some of the singing is distinctly small-scale. Even without a production that sinks under the weight of its own implacable earnestness, this would be yet another completion of Zaide headed straight for the discard pile.

Until 26 June. Box office: 0844 412 4300. Then touring. See details.