There's an irony to Guards at the Taj at the Bush. After a £4.3 million upgrade, the theatre reopens with a play probing the cost of a beautiful new building. In fairness, the Taj Mahal was rather more expensive (around £700 million in today's terms), but then it is rather more beautiful. The Bush's new wing is nice and all, but it's no Taj Mahal. It's not, as the poet Rabindranath Tagore put it, a "teardrop on the cheek of history."

The Mughal emperor Shah Jahan had the Taj Mahal built for his wife – the world's most beautiful building in memory of its most beautiful woman. It took 20,000 men 20 years to build and, to preserve its status, Shah Jahan ordered every one of their hands be cut off lest they ever build anything so beautiful. That job falls to his imperial guard: 40,000 amputations. One chops. Another cauterises. Hands pile up in baskets.

It's folklore – in reality, they got pensions for life and were forced to retire – but the American playwright Rajiv Joseph turns the tale into a fable for our own times. We are, he implies, living in a new Mughal era: the one per cent and the rest of us.

Two guards stand with their backs to the new Taj, forbidden to turn around and take her in. Danny Ashok's Humayun is cheerily obsequious, a stickler for duty; Darren Kuppan's Babur, dreamy and gently defiant. They make a tight double act, fidgety but fond, with time to kill. Both are desperate for a promotion to get closer to the emperor and, ahem, his harem, so when they finally sneak a look, it's a big risk in itself.

The sight stops them still. Imagine seeing the Taj for the first time, with no preconceptions, on the day of its unveiling: perfectly pristine as it soaks up the sunrise. Their swords drop to the floor. They hold hands without thinking.

Fast forward and they're drenched in blood, near catatonic. After a day of amputations, Humayun's blind and Babur can't drop his sword. The horror of what they've seen – what they've done – outweighs any beauty in the world. Whatever the reward, the guilt stains them for life.

Guards at the Taj turns over questions of inequality and aesthetics. For these two, beauty isn't just out of reach – the preserve of the emperor alone – it breaks down altogether, and Joseph artfully articulates the way life wears us down through compromise and complicity. The play's comic gloss – the double act pitter-patter of two endearing friends – masks a heavy heart.

It's a riddlesome play that owes, and acknowledges, a debt to Tom Stoppard's Rosencrantz and Guildenstern are Dead. Joseph gives us two more forgotten men, friends and colleagues, pondering logical conundrums as they reminisce, dream and joke. Lightness makes it all the more disconcerting and, while Ashok and Kuppan play it with deft street-geek charm, Soutra Gilmour's blood-encrusted design makes all its awfulness count. It's Morecambe and Wise in hammer horror; the Two Ronnies in hell. We smile as our stomachs churn.

For all its gore, though, Jamie Lloyd's staging tunes into its beauty too. Richard Howell slings moonlight and sunbeams across the space, and George Dennis conjures natural majesty through sound, as birds shoot over our heads. It's human too. The way Ashok cares for a broken-down Kuppan, cradling and washing him, peeling off blood-sodden clothes, might surpass even the Taj Mahal itself.

In that, Guards at the Taj becomes less ironic, more a mission statement. No matter how beautiful a building, it's humanity that counts – but only if it throws off a system built out of inequality.

Guards at the Taj runs at the Bush Theatre until 20 May.

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We sat down for 90 seconds with Guards at the Taj playwright Rajiv Joseph: