I have quite a flexible attitude towards Bent, Martin Sherman’s stark 1979 narrative about the Nazi persecution of homosexuals in Berlin and, after the purge of the Brownshirts in 1934, in Dachau concentration camp. On the one hand, it's as terrifyingly banal as it is terrifyingly sentimental. On the other, its dramatic crudity and flagrancy is exactly the point.

Daniel Kramer’s revival does not pull its punches in this respect. The play is presented with a defiant flourish, smoke and flame bursting through fissures with a Wagnerian soundtrack suggesting the apocalypse, and Alan Cumming’s Max slithering to his destiny with a devious stickability that just about stops short of breaking out in a chorus of Gloria Gaynor’s “I Will Survive”.

The role of Max was created by Ian McKellen, first with Tom Bell (whose death was reported in a curtain speech by the author) as Horst, his lover and nemesis in Dachau, and later, in the 1990 National Theatre production, with Michael Cashman. On that second occasion, public attitudes towards homosexuality, complicated by some apparently hostile Government legislation, had taken a turn for the worse, so the necessity of the revival outweighed all misgivings about its mawkishness.

Now, with the “new look” Conservative party making statements about respecting marriages between all gender types, the crusading elements give way once more to the theatrical. Alan Cumming, most mercurial (and funniest) of Hamlets before he gave full rein to his sleazier side as an unforgettable, bare-buttocked Emcee in Sam MendesCabaret revival, returns from New York to update us on his maturing acting ability.

In the Berlin scenes – where Richard Bremmer plays a hauntingly cadaverous transvestite singer at his dressing table - Cumming is full of devilry, reclining lasciviously on the sofa while trying to remember who (and how many) came back to the flat he shares with Kevin Trainor’s devoted Rudy. Gosh, I thought, Cumming’s got a hairy bottom, until I realised those were big dark bruises he was flaunting like battle scars.

Things become tenser in what remains my favourite scene from the play, the park bench encounter with “Uncle” Freddie (beautifully done by Hugh Ross) who arranges alibis and travel documents while casting a furtive eye in the direction of an off-stage policeman. Then things become ugly as the Nazi thugs take over and, on the train to Dachau, Max is compelled to complete the murder of Rudy by hitting him with a truncheon. That obscenity is compounded by his confession that he had sex with a dead teenaged girl to “prove” he isn’t gay.

What are we supposed to feel? That circumstances justify Max’s actions? That we would do the same in his place? Aesthetic revulsion becomes confused with intellectual sympathy, and just as we have entered a place of no return, the second act friendship of Max and Horst, lugging their stones in the bleak compound, restores our faith in humanity.

Tom Bell’s Horst was an eloquently understated foil to McKellen’s obsessive survivor, the man who exchanges a pink triangle for a yellow star because Jews are given meat in their thin gruel. Twenty five-year-old Chris New is a busier Horst, doing much more “acting” to convey his decency. But the scene where the two men have telephone sex without the telephone, simply standing next to each other on a bare stage, leaps at you with undiminished fervour and power.

- Michael Coveney