On another night, this might have been a five star review. As it happens, it isn’t. How do you review a show that changes completely and wholesale with each performance? Such is the promise of Showstopper: to use audience suggestions to transport us to a newly-imagined world full of musical improvisation every evening, with a little help from the moments of spontaneous comic injection that flow inevitably from it.

One of the richest treats for a reviewer, of course, is that I need not worry about giving away any spoilers, because the show really is totally improvised and totally unique with each incarnation. I am fortunate enough to have borne witness to this fact first hand, having been to see Showstopper during its summer run at the Edinburgh Fringe, which in the last seven years of the show’s existence has been its launch pad, in recent times securing it a run in the West End, as well as on tour, which brings it to our very own Arts Theatre.

On that first occasion, it was with much reluctance and heel-dragging that I was shepherded, along with my distinctly low expectations, into the theatre to sit through 90 minutes of what I thought would surely amount to a cringe-inducing on-the-wing hodgepodge of Broadway rip-offs. On that occasion, I and my preconceptions were bowled right over; so slick were the musical numbers and the transitions, so skilfully and humorously were the scenarios drawn up, that the standing ovation was the least the audience could do.

From the serene Ancient Roman setting on that occasion, to the infernal Circles of Hell on this, the change could hardly have characterised the marked difference in performances better. My burgeoning expectations of Showstopper on tour here in Cambridge were deflated in stages.

After some truly awful suggestions from the audience (who wants to watch a musical set on the Cambridge Station cycle path?), we settled on the theme for the night, guided by buoyant, bearded compère and co-founder of Showstopper Dylan Emery: our musical Hell Dorado, featuring songs in the style of Rocky Horror, Starlight Express and Wicked, amongst others, joined the list of previous titles including the religio-gangland Pope Fiction and FIFA-inspired improvisation Blat Out of Hell. Hell Dorado quickly developed into Mephistopheles’ (‘Mephy’s’) inner struggle between staying in hell as Lucifer’s head torturer and ascending back to heaven with her newly hell-bound lesbian ex-lover, so powerful are the redemptive effects of her rekindled love.

The trouble was, that was as far as the development went. Don’t get me wrong, I wasn’t by any means expecting a great Shakespearean melodrama in six acts, but when your scenario stays essentially the same for an hour or so of musical enterprise, things are going to get repetitive. So it proved, as distinct numbers became few and far between, both before and after the interval, after which time one was willing the performance to pick up a bit more cohesion (to little avail). The highlight of the show, both in Edinburgh and tonight, though, was the band, who did their utmost to carry the evening through.

Actually, to some extent, they were so impressive in adapting deliciously to each musical styling – from Flamenco to Phantom of the Opera – that they succeeded in transporting us into the hellish settings on stage and pouring consistent vats of energy into the show. Even they were rather defeated by the Bollywood set piece, however, which laid bare the shortfalls of the cast members and fizzled out.  I’m not usually in the business of singling out any one actor for particular blame, as it isn’t fair or typically representative of the whole run of the production, and I’m sorry to have to do it, but Philip Pellew did just seem to get in the way a good deal, both physically and with his improvised plot lines; never was this more evident than in a number on the hideousness of clichés, which he was leading, wherein the only adage he could must was a lame ‘grass is greener’ and had to be bailed out by his co-stars when he started struggling. Notably impressive on the night, however, at least in parts, were Ruth Bratt (a Showstoppers stalwart), Andrew Pugsly and Lauren Shearing, who seemed to be sharing a common wavelength more often than not.

This all sounds rather harsh so far, but none of this criticism is to say that Showstopper was not tremendously good fun. Most of the performers gave us some marvellously witty one-liners at some point or other, and there were more than enough catchy chorus lines to provide whistling ammunition for a few days at the very least. Similarly, the lighting deserves applause, as it was, in a number of places, inspired; one instance that springs immediately to mind is the sudden lighting of a backdrop of stars during an especially poignant ballad. I always had a smile on my face over the course of the evening, put there by the great, continual flashes of humour in situations and lyrics alike, which carried one through the more disappointing times. During these lulls, though, you could feel the audience willing the show to stay afloat and not fall to bits, rather reminiscent of that feeling one senses in one’s stomach while watching a school production: hoping desperately, vicariously, bodily, that it’s all going to be okay.

The overwhelming impression that I would like to give of Showstopper, despite such undertones of awkwardness in the specific production I witnessed, is that it really is worth going to see, because what matters much more than the plot, how repetitive the songs might be and whether there are enough syllables to each line of the verse, is how much you’ll enjoy yourself. It is a lottery, no doubt, as you could always go on something of a slow night such as tonight, where not everything clicked neatly into place as it did when I first saw the show in Edinburgh, but the odds are stacked in your favour: it will likely blow you away, but, if not, you’ll probably enjoy it anyway. After all, alongside the opportunity to be ‘part of the show’ by participating as in no other production, it’s the contingency that renders this kind of improvised experience such a thrilling ride.