Gone with the Wind, New London, London<br/>Harper Regan, NT Cottesloe, London<br/>One Step Forward, One Step Back, Anglican Cathedral, Liverpool

'Give me strength!" cries Scarlett O'Hara. "As God is my witness, they are not gonna to lick me. I'm gonna live through this!" Frankly, enduring Trevor Nunn's big, new, seemingly interminable musical adaptation of Gone with the Wind, I was muttering much the same.

Novelist Margaret Mitchell's saga of love and the American Civil War – immortalised in the classic Hollywood movie – has epic sweep, of course. It may, indeed, be worth revisiting her portrait of over-confident Americans – in this case, the white supremacists and wealthy plantation-owners of the Deep South – marching to war then seeing their cosseted world go up in smoke. A topical moral lesson may even lurk in Scarlett O'Hara's journey from pampered brat to determined survivor in hard times, striving to retain her family home.

To give Nunn's designer, John Napier, his due, the set creates a sense of both panoramic breadth and intimacy. A wide cyclorama-style sky arcs over a wooden verandah of faded grandeur, with weathered picket fences, old Confederacy flags and slave auction signs encircling the auditorium's balcony.

However, the dramatisation is inept. With book, lyrics and score by a Dr Margaret Martin who has no musical track record, the storytelling feels, paradoxically, long-winded (at over three-and-a-half hours) and ludicrously rushed. It lurches forward spasmodically, in such a way that military leave for Scarlett's first sweetheart, Ashley, looks more like a mini-assault course: up the verandah steps, about turn, and down again. With more than 70 minor characters, you don't care about any of them either.

Nunn possibly thought this show could be another Les Mis or Nicholas Nickleby, but his stylistic use of physical theatre is tired and patchy here – a few mimed doors and some extras pretending to be a horse. Worse still is the poor acting in the leading roles. Darius Danesh as Rhett has rakish swagger and a mellifluous singing voice, but when in a passion he becomes ridiculously melodramatic. He kisses Scarlett like a frenzied dental hygienist, trying to scrub her teeth with his moustache.

Meanwhile, Jill Paice's Scarlett is pretty yet not engaging, with strident singing and no genuinely fiery sexual magnetism. With folk tunes spoilt by sugary orchestrations, Martin's unmemorable songs wreck any dramatic momentum. They are the theatrical equivalent of speed bumps. You can almost hear the show's chassis grating to a halt. Only the Gospel numbers by the O'Haras' servants – especially Natasha Yvette Williams's splendid Mammy and Jina Burrows's Prissy – rise above all this as they look forward to emancipation.

The titular heroine in Simon Stephens's Harper Regan is teetering on the verge of a mid-life nervous breakdown. This is a quietly strange and harrowing portrait of a wife and working mother, with a dark family secret that has estranged her from her own parents. Going off the rails when her father dies, she finds herself in a hotel room with a womanising stranger – a situation that makes an intriguing parallel with Lucinda Coxon's Happy Now, currently also in rep in the Cottesloe.

Admittedly, Stephens's opening scene seems awkward, with Michael Mears not fully convincing as Harper's ghastly boss, blathering about porn. It takes a little while to adjust to this writer's curious twilight zone between the everyday and the borderline crazy. But Marianne Elliott's cast become enthralling, sliding between comedy, serious menace and tenderness. On one level, this play contemplates our contemporary taboo-breaking – particularly regarding internet porn, sexual aberrations and pernicious racism. On another level, Elliott brings out how Stephens' characters struggle to repress their hurt and shame. Lesley Sharp's bony, haggard Harper is riveting, shaky and dangerous with huge pained eyes. Nick Sidi as her husband has his own closing moment of devastation. Jessica Raine is superb as Sarah, their stroppy teenage daughter: brilliantly realised, troubled but coping, never exaggerated. The mother-daughter confrontation a generation up is ferocious too. Susan Brown is on top form as Harper's estranged suburban-genteel mum, explosive under attack.

Finally, to Liverpool – European Capital of Culture '08 – for dreamthinkspeak's site-specific piece in the Anglican Cathedral. One Step Forward, One Step Back is an extraordinary promenade event, combining installations and actors, where you are left to wander alone right up through attics and a vast bell tower to the open roof where the city's golden lights seem to stretch away to infinity.

This venture does not get off to a flying start. There are some unsophisticated vignettes and video projections featuring slave-driven Father Christmases and modern Adams and Eves staring through the bars of supermarket trolleys. But soon you discover other fabulous installations, including twinkling miniature Alpine towns and huge avalanches of snow in the attics. The higher you climb, the more haunting and unexpectedly heartrending this piece of work becomes. Loosely based on Dante and William Blake's verses ("Till I have built Jerusalem..."), it muses on how we have lost – or might strive to regain – a state of bliss.

Dreamthinkspeak has grasped that there is something ironically, fantastically infernal about this great church with its maze of narrow passages and spiralling stairwells, its giant cogs and wheels and sheer brick walls. And yet the company simultaneously inspires an intense sense of spiritual yearning, an effect more moving than a hundred sermons. The composer Max Richter's ethereal music – a high, angelic but ever-falling scale – draws you on, mesmerised. At one point you round a corner to find yourself, breathtakingly, at the top of the nave, way up by the rose windows. You are gazing down on the shadowy choir and altar. There an azure-robed woman – like a distant hazy vision, emanating her own soft glimmer – walks slowly away from you. Finally she turns, as if she senses you, just raising her hand as the light fades. Unforgettable.

'Gone with the Wind' (0870-040 0046) booking to 27 September; 'Harper Regan' (020-7452 3000) to 15 May; 'One Step Forward, One Step Back' (0844-800 0400) to 10 May

Top of the flops

'The Lord of the Rings', right, cost a record £12.5m to stage in the West End last year. It has now announced its imminent closure without breaking even. 'Behind the Iron Mask', an absolute howler based on a story by Alexandre Dumas, was given the chop two days after opening in 2005. But the record shortest run is held by 'Oscar Wilde: the Musical', penned by radio presenter Mike Read and silenced after just one night in 2004.