One answer is obvious: Maria Björnson’s stunning designs. The story, based on Gaston Leroux’s novel, begins on the Paris Opera’s stage in 1911.

The building is dusty and neglected, the stage festooned with great swags of grey fabric. Old theatrical props are being auctioned off. Then, as the auctioneer announces the final lot — the opera’s enormous chandelier — the dustsheets fall away, the scene seems almost to dissolve and the theatre travels back in time to 1881 before our very eyes. It’s a vision of gaudy grandeur: gas lamps flicker, red velvet glows in the light and plaster nymphs and satyrs frolic on pillars.

The opera is dogged by misfortunes that are rumoured to be the work of the Phantom, a malevolent figure who haunts the theatre. This sinister creature has become obsessed with Christine Daaé, a pretty chorus girl. But only cadaverous Madame Giry, the formidable ballet mistress, knows the terrible secret that lies behind the mask the Phantom wears.

The musical’s best sequence occurs when the Phantom leads Christine through labyrinthine passages to his lair beyond a hidden lake underneath the building. As their boat glides over the misty water, surrounded by flickering candles, the effect is breathtaking. The problem is that this spectacle takes place early on and nothing afterwards can really compete. Harold Prince’s direction is slick and Gillian Lynne’s choreography effective, but Lloyd Webber’s music is often trite and dated.

There’s little depth to any of the characters, either, yet the musical’s central relationship intrigues. For while Christine is Beauty to the Phantom’s Beast, she is haunted, not only by the opera ghost, but by the memory of her dead violinist father. This lends a pleasingly perverse Freudian twist to the erotic tension between the two characters, and the performers John Owen-Jones and Celia Graham make the most of it.

Not only are they both excellent singers, they are fine actors too. Graham brings both passion and a childlike neediness to her portrayal of Christine. Owen-Jones uses his rich, powerful voice to great effect, crooning one moment, spitting bitterness the next.

But the real star of the show is Björnson’s work. The designer died earlier this month, aged 53, and if Phantom has stood the test of time, it’s because of her. It is not the music audiences will remember; it’s the opera’s chandelier swinging crazily over the stalls, or a masked figure crouched menacingly over a gilded angel. And for those images alone, this show is still worth seeing.

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Tomorrow: Benedict Nightingale revisits The Woman in Black