One of the perks of writing a review of an improvised musical comedy is that nobody can accuse you of peddling spoilers. The musical I saw, East Side Story – a romantic epic about Jon Cri (Julie Clare) a North Korean dictator’s daughter and her whirlwind love affair with Kimberly (Pippa Evans), a young woman from Birmingham who loves biscuits – will never again play the stage of the Udderbelly.

Spoiler #1: it’s all fine in the end; everyone kisses.

The cast, a team of six from a revolving talent pool of thirteen performers is headed up by Dylan Emery, who plays the “author” of the piece, corralling the audience’s suggestions for plot and musical styles into a vaguely structured narrative. It’s fun watching Emery shape the audience’s musical suggestions into set-pieces: the montage, the love duet, the comic ensemble number, and the rest of the cast are more than capable of rising to the challenge.

Most effective is the final number, a sort of Sunday In the Park With Kim Jong-il replete with swooning harmonies and twisty Sondheim-brand lyrics by Lucy Trodd and Oliver Senton.  Vocally, there isn’t a weak link among the cast and their accompanist Yshani Perinpanayagam is astonishing in her ability to conjure up the tone of any given musical without simply aping the score. Her Legally Blonde-ish ode to totalitarianism titled Freedom Stinks was especially spry and impressive.

However, this brings me to Spoiler #2: the show just isn’t that funny.

The trouble with a deftly executed harmony is that it hides a multitude of sins. The performers are so game and their facility with musical style so fluid, the effect on the audience is spellbinding – we are watching a group of highly skilled professionals doing something that not many people can do and it’s exhilarating. But the comedic tropes are panto-inflected (of the Oooh Saucy School of same-sex kissing), there’s little attempt at lyrical inventiveness, and the laughs, when they do arrive, come almost solely from physical comedy. Only Trodd, who has a fine line in Eva Braun-based mime, and Justin Brett, with an impassioned Ronan Keating impression, felt as focused on the laughs as the musicality.

That said, improv is high-wire stuff, hugely dependent on its audience, and your bespoke musical results may vary. Just do yourself a favour when you’re asked to yell out suggestions: skip the Miss Saigon stuff and head straight to Avenue Q. It’s called musical comedy for a reason, y’know.

Showstoppers! The Improvised Musical. Udderbelly Festival at Southbank Centre, London SE1 8XX. Every Sunday, 19th April – 12th July, 7.45pm. £16