A scene from Showstopper! The Improvised Musical
A scene from Showstopper! The Improvised Musical
© Geraint Lewis

"Please tweet us your suggestions in the interval for what should happen in Act Two." This, in the programme, is a West End first, and so is a ten-week West End season of the deliriously inventive, improvisatory Showstopper crowd; that means, eighty brand new musicals, one each performance, developed from suggestions for setting, story and musical styles taken from the audience.

So, at yesterday's matinée, we plumped collectively for "Puck Off," a fairy grotto scenario in the Belfast hills in which the lead fairy had to win her wings in an act of bravery with musical numbers referencing Wicked ("Defying prophesy"), Les Mis ("I had a friend"), Avenue Q and Stomp, this last a glorious ensemble number in the dragonfly shed at the bottom of the garden of a big house.

I hadn't seen Adam Meggido and Dylan Emery's improvised gang show for a few years (they've been in business since 2007) and frankly feared staleness and over-reliance on too many familiar musical theatre tropes. But, hallelujah, their spirit is undaunted and their genius undimmed, even though the format is the same: Emery, seated at the side of the stage with his acoustic guitar, is on the telephone pitching whatever show we are about to make up to a producer called Cameron.

The only implausible element is that Cameron likes everything Emery suggests! And of course love wins through, evil – in the shape of Andrew Pugsley's hilarious "Rock of Ages" Puck, who has possibly killed the Fairy King and seduced his widow – is punished, the fairies break out in Riverdance mode at the fairy wedding (rhymed with bedding), wings are won.

Interval tweeters pined for the Jersey Boys and a Gilbert and Sullivan patter song and got both, brilliantly. There's an interlude in the big house (before it catches fire) where tea is taken and Meggido's cringing manservant Plankton does a One Man Two Guvnors turn that might have been shoe-horned in; but probably not, as we are also suddenly assailed with responses to earlier call-outs for Ann Summers with strange fairy toys and liquids.

Emery freeze-frames the action when he feels a narrative surge, or special kind of song, is needed and the onstage musicians follow his cue. But the main impetus comes from the cast – in my show, stalwarts Meggido, Ruth Bratt and Pippa Evans were outstanding, and Justin Brett, looking something like Alistair McGowan, particularly adept at nudging narrative corners and finding glints of comedy.

Good new British and American musicals must come from elsewhere. Meanwhile, Showstoppers revel in the pleasures and pitfalls of the back catalogue, with satirical ingenuity and a fully tuned critical alertness and mimetic merriness.

Showstopper! The Improvised Musical runs at the Apollo Theatre until 29 November.