Fill the stage with children, the auditorium fills in turn with relatives and friends, and you bring the house down. It's an old story given a fresh twist at this afternoon Prom by professional levels of training: 40 young singer-actors from a two-week residential course courtesy of BBC New Talent, backed up by massed youth choirs. And they delivered on the world premiere of The Water Diviner's Tale with a power and precision that was a match for the BBC Concert Orchestra. Composer Rachel Portman and writer Owen Sheers could have given them any old rubbish to deliver, and they would still have thrilled.
On Portman's part, at least, the goods were top notch. Her "dramatic musical piece for people of all ages" – fancy name for an oratorio, really – began with a storm of nagging strings and growling brass, piled up the voices as children ran to the stage from the hall, and kept up its energy and its directness of melody through a sequence of affecting and satirical numbers. It wasn't a very scary storm, but it was a big sing and it set the show off to an electrifying start.
Then the momentum went and younger kids in the audience were starting to fidget. You can blame it on the Water Diviner, but that would be unfair to Nonso Anozie's prowess and hall-filling presence in the narrator's role. Sheers made him a John the Baptist of climate change, a lone figure bringing the truth to a world that wouldn't listen. Only these children, refugees from storm and flood trapped in London's Superdome, heard him out and vowed to save the world.
If John the Baptist had been half as preachy, he'd never have made it into the Bible. Anozie harangued any human activity that smelled of smoke. Dodgy scientists and businessmen were wheeled on as he told the children about a mysterious boy, the Fuel Diviner, who once dreamed up the use of natural resources to better the human lot, and turned out to be the Water Diviner before he grew up.
Only a parody weather forecaster with strap-on smile sweetened the pill. Historical motives were rewritten so that greed became the driving force, to such manipulative effect that you started feeling responsible for the entire career of the motor car. The cause is ill served by such a disingenuous harangue, never mind the audience. "Polar bears are drowning," moaned the chorus – never mind that polar bears can swim, as one chorus member points out in an easily available blog.
Some of the Songs of the North, East and South were more truthful, but too late to make you trust this Diviner character. Even the final chorus of resolve didn't come up with a melody to lift things. Still, a strong score and a terrific ensemble performance under the stage direction of Denni Sayers with David Charles Abell conducting.
BBC Proms continue to 8 September (020-7589 8212). Visit independent.co.uk/promcast for exclusive daily podcasts and listen online to highlights from the previous night's PromReuse content