What enthuses Ellen about Joseph Wright of Derby's famous painting An Experiment On a Bird in the Air Pump is that it places "a scientist where you usually find God". Ellen is a geneticist working within reach of the prediction of individual genetic codes, and thus, perhaps, their prescription. As she gazes, the tableau vivant of Wright's painting comes to life and the white dove that has pawkily watched us take our seats disappears from its glass dome as the experiment is completed.

Such is the brilliant beginning of Shelagh Stephenson's ambitious new play An Experiment With an Air Pump. The figures in the painting resolve themselves into Joseph Fenwick and his household. Fenwick is an 18th-century progressive in the mould of Joseph Priestley, a scientist who, at the end of 1799, envisages a future of ever-increasing knowledge bringing infinite well-being and democracy. The flavour of this Enlightenment enthusiasm, cross-cut by the scepticism of Fenwick's anti-scientific wife Susannah, is swiftly and funnily given as he and Roget (of Thesaurus fame) consider papers for the Literary & Philosophical Society: a history of whelks "fails on the visionary count", but "Is Progress an Illusion and the Past a Myth?" sounds interesting.

The play crosses back and forth between Fenwick's world and our own as Ellen and her husband Tom debate whether she should accept a lucrative post with a genetic engineering company. Stephenson is concerned to point the continuity of the scientific problem as she sees it; the inappropriateness of applying scientific disinterest to the tumble of variables that constitute the human affections. A scientist might groan at this familiar antithesis, and indeed the extremes of the scientific spirit, the young physician Armstrong in 1799, and Kate the biotechnical entrepreneur in 1999, are drawn as callous stereotypes. But for Tom Mannion's fine portrayal, Armstrong would be a positively Jacobean villain, seducing with tragic consequences the hunchbacked servant-girl Isabel to enable his study of unusual anatomy.

This disaster is part of what teaches Fenwick that the world cannot be as cheerfully and comprehensibly described as he has thought. The other part is the emotional wreckage of his marriage and the tumult of his daughters' ambitions and amours. Instead of marching god-like into the lucid new century, the scientist finds himself "groping blindly over the border in bewilderment".

Shelagh Stephenson has written a play teeming with interest, humour, eloquence and, above all, ideas. Matthew Lloyd, designer Julian McGowan and an excellent cast have realised it generously and clearly. At times though the ideas are doing all the walking and talking - especially in the 1999 sequences. The differences between Kate and Tom are vital but exist only as an occasional debating exchange, and Ellen herself is insufficiently developed. Especially since the gender roles are reversed from 1799 - David Horovitch plays Fenwick and Tom, Dearbhla Molloy Susannah and Ellen - it is a pity the two parts of the play are so unbalanced. Nevertheless it's not often we see a new play with this much energy, variety and intelligence.

To 7 March. Box office: 0161-833 9833