Laura Wade’s adaptation of Sarah Waters’ 1998 novel has a big heart, offers a feminist message and is staged by Lyndsey Turner with a witty inventiveness that makes up for her Barbican Hamlet. But it is possible to have too much of a good thing and, after three hours, you become aware of the problems of putting a big, picaresque novel, once described as a Sapphic Moll Flanders, on stage.

Wade ingeniously frames the story by presenting it through the eyes of a gavel-wielding Victorian music-hall chairman of the kind made familiar by TV’s The Good Old Days. This pays off beautifully in the first half, which is a hymn to theatre. We see how Nancy Astley, a Whitstable oyster-girl, falls rapturously in love with a male impersonator, Kitty Butler, and moves from being her dresser to her trousered co-star in the guise of Nan King.

All this is rendered with exuberant vivacity, as modern pop-songs are re-arranged by Michael Bruce in Victorian style, and Nan and Kitty express their sexual ecstasy through an aerial ballet reminiscent of the Lyric’s staging of Angela Carter’s Nights at the Circus. Even when Nan discovers Kitty’s duplicity, the idea of presenting her life as if it were a music-hall saga is maintained. Lizzie Clachan’s designs are a series of theatrical backdrops: in one especially clever section, when Nan passes herself off as a male renter, the sexual organs she blows and fondles are pipes and bells protruding through the peepholes of a seaside photo-board.

Brilliant as Waters’ book is, it loses some of its zest when Nan becomes the plaything of a wealthy lesbian and eventually the lover of an East End social worker. The character of Nan becomes the excuse for a tour of the Victorian sexual scene and Wade’s adaptation cannot disguise the diminishing tension: the story becomes as much a study of class as sex, but the contrast between Diana Lethaby’s snooty upper-class Sapphists and the homely warmth of Bethnal Green socialists is a bit too good to be true.

There is, however, one tremendous moment late on in a long evening: that is when Nan discovers her political voice and hymns the idea of gender equality to the tune of Lee Hazlewood’s These Boots Are Made for Walkin’. This is also the high point in a fine performance by Sally Messham who, in her professional debut, displays a consummate assurance and a perky mischief that put me in mind of the young Barbara Windsor.

In this ensemble show there are stylish contributions from Laura Rogers as the cross-dressing Kitty, Kirsty Besterman as the dildo-worshipping Diana and Adelle Leonce as a hospitable charity worker. David Cardy also captures well the leering voyeurism of the chairman through whose male eyes the story is told.

It all makes for a good night out that offers a rousing tribute to feminist principles. But it strikes me, not for the first time, that episodic adventures spread over a period of time work even better on the page than the stage.

• At Lyric Hammersmith, London, until 24 October. Box office: 020-8741 6850.