How to execute Mozart's unknown ending

Mozart left his opera Zaide uncompleted and with his heroes facing death. Ian Page explains how he finished what the composer started

How do you complete an unfinished Mozart opera? Or perhaps the first question should be, "Why complete an unfinished Mozart opera?" Schubert's two-movement "Unfinished" Symphony is one of the most popular works in the classical music repertory, and indeed, attempts to create third and fourth movements from the composer's surviving sketches have generally proved unsatisfactory. Deryck Cooke's completion of Mahler's Tenth Symphony has enjoyed considerably more acclaim but he did have extensive sketches by Mahler on which to base his work. On a more modest scale, I recently composed new recitatives to complete the score of Thomas Arne's Artaxerxes, for a new production which I conducted for the Royal Opera last year, but the original libretto has survived complete, so the process was a relatively straightforward and uncontroversial one.

Mozart's Zaide, however, is a much more complicated jigsaw. He wrote two acts of the opera, including the ravishing soprano aria "Ruhe sanft", before breaking off when he was commissioned to write Idomeneo. One thing led to another and he never returned to Zaide, although it is clear from his letters that he held the work in high regard. Unlike Schubert's "Unfinished" and Mahler's Tenth, there are no sketches to refer to, and none of the spoken dialogue or the text for the final act has survived, so we don't even know how Mozart intended the story to finish. The obvious option, or at least the easy one, would be to present a concert performance of the 15 numbers of Zaide which Mozart completed.

But Zaide was written for the stage, and vocal or operatic music is fundamentally different from orchestral or instrumental music. The moment sung text is involved, the music ceases to be purely abstract, and communicates specific meaning, emotion and character. The music is inextricably wedded to the text and the drama, and therefore has a decisive narrative element. Mozart's operas contain glorious sequences of beautiful music, but above all they tell a story.

Zaide is a serious, powerful tale exploring the themes of tyranny, imprisonment, the abuse of power and the power of love. The prisoners Zaide and Gomatz have fallen in love and attempted to escape, but they are recaptured and sentenced to death by the tyrant Soliman. The final number which Mozart composed for the opera is a remarkable quartet in which the two prisoners and their ally Allazim beg for mercy. Their voices blend mellifluously as they yearn for freedom, but Soliman's unforgiving voice simultaneously strikes a harsher tone as he refuses to listen to their pleas. They are to be executed.

In Mozart's hands, such a dramatic situation makes for wonderful opera and an intriguingly poised ending to the second act, but this is obviously not the ideal point at which to send the audience out into the night. Drama is about conflict and resolution, and Mozart was particularly concerned in his operas with the redemptive power of drama and music. We do not know how he intended to finish Zaide, but we can be sure that he intended the work to be completed and staged.

He was already a fully-formed and experienced artist of 23 when he started writing the work, as is evident from the consistently high quality of the music, and there are numerous aspects of Zaide which I find particularly intriguing. Firstly, it dates from a very important phase in Mozart's emotional development: not only had he recently fallen deeply in love with an acclaimed young singer, Aloysia Weber, who broke his heart by rejecting him, but also his mother had died while chaperoning him on a visit to Paris. Secondly, Mozart embarked on Zaide without having received any commission for it. This may not seem remarkable to us nowadays, but at the time it was almost unheard of for a composer of Mozart's stature to embark on an opera without first knowing that he would be paid for it and that it would be performed. Finally, Zaide was written not in Italian, the traditional language of opera, but in German, the language of the composer and his audience. All of these aspects combine to create the impression of a very personal and significant work, and yet, for understandable reasons, Zaide remains very little known and rarely performed.

For several years now I have been developing the idea of creating a completion of Zaide, and later this month this new version will finally be performed for the first time. In keeping with Mozart's wish to present the work in the vernacular, our version will be in English, with a libretto by two of the UK's leading young writers, the poet Michael Symmons Roberts and the playwright Ben Power, and our remarkable director, Melly Still.

To complete the score I have selected a total of six pieces – four vocal plus an overture and a prelude to the new third act – by Mozart (I could think of no composer who could do this job better); my main criteria were that the additional music should be of outstanding quality (even by Mozart's standards), should nevertheless be relatively unfamiliar, and, in the case of the vocal music, should blend organically and not need its original meaning and context radically reworking to fit our purposes. The whole viability of the project was dependent on being able to find suitable music through which to chart a completion of the story, and it took a long time to establish the structure of our final act.

Because we have no information on how Mozart intended the opera to end, the work has necessarily been more creative than recreative, more imagination-led than scholarly, and more true to the spirit than the letter. Of course our version is not the only solution, but given that Mozart wrote such a fascinating beginning and middle, the temptation to create an end proved irresistible. The result, I hope, will be a visceral and dynamic piece of theatre which draws its inspiration from the legacy and the values of one of the greatest artists ever to have lived.

'Zaide' opens at Sadler's Wells, London EC1 (Sadlerswells.com) on 24 June and then tours to Sheffield, Bath and Buxton.