There’s a powerful school of thought that says musicals will be the branch of theatre that best survives the credit crunch. Their uplifting, feel-good spirit, so the reasoning goes, will keep persuading punters to part with hard-earned cash. The brains behind this theory have obviously never seen Rue Magique.

There have, of course, been unlikely subjects for song and dance shows before but none could even begin to rival a narrative that centres around the rape of a child prostitute on her 13th birthday.

To say this is a moral morass would be one of the great understatements of the theatrical year. We emerge feeling shaken and grubby, and also guilty for finding so much to admire about Lisa Forrell’s production. It’s slick and well choreographed, and Brett Kahr’s accomplished music and lyrics are performed by fine voices.
Yet a reminder of the disconnect between form and subject matter is never far away. The stirring gospel number This House is Clean is about the importance of disinfecting a brothel. The Viper’s Tale, performed by three of the brothel’s clients, has all the makings of a cult hit, as the sad sacks sing jauntily of how It sucks when you have to pay/ When you’re fat, masochistic or gay.

How we’d laugh, if only we hadn’t seen the terrified little girl they were lining up to screw.

Nadia Di Mambro sparkles with innocence as Sugar, the daughter of fearsome, agoraphobic brothel-keeper Desdemona (compelling Melanie La Barrie). As she knows how to read and write, Sugar doesn’t go to school any more, and her only meaningful contact with the outside world is with kindly, smitten Rem (Terel Nugent), who works in the local south London corner shop. He’s undoubtedly a good man. Sugar is even more undeniably three years below the age of consent.

How to round this off? With a glimmer of hope, for sure, but such an unholy mess cannot be made right with one final power ballad, as Kahr and Forrell try here. It’s murky, troubling stuff, which would make reading the balance sheet of Icesave a hoot in comparison.

Until 7 December (0870 890 0149, www.kingsheadtheatre.org).