Successfully improvising a show requires the cast to think on its feet, work together easily and anticipate each others’ lines. Improvising a musical requires the cast to do all of this whilst singing and dancing. Improvising a musical to West End standard requires harmonies, clear plotlines, witty comedy and a full band to support you. This doesn’t sound simple at all; Showstopper! make it look effortless.

Narrator Sean McCann seeks inspiration from the audience and eventually constructs a musical called Class-Y Title, set in a staff room of school. It incorporates styles from classics such as West Side Story, Rocky Horror Show and Cats. The story unfolds over the course of a school year –Science teacher Maureen (Pippa Evans) and Home Economics chef Jacques (Justin Brett) have just begun dating; PE teacher Lydia (Nell Mooney) is pregnant and Literature teacher John Johnson (Andrew Pugsley) is busy arranging the school play whilst grappling with his feelings for the principal Nancy (Ruth Bratt). Unfortunately she is just too focused on her job to pay him any attention.

McCann has an uncanny ability of freezing scenes at poignant moments in order to incorporate a new musical style – it keeps everyone engaged and on their toes. He also gives the audience an opportunity to throw in some lines to a scene and so Mr Johnson (Pugsley) is forced to confide in Lydia about his feelings for Nancy – “Your arse is like an onion, it makes me want to cry” and “I just want to bang your two erasers together.” McCann has absolute control and exerts this to full effect, as Maureen (Evans) finds to her dismay. She comes on stage at the wrong moment and so sings about “a morally and structurally applicable class experiment” in the style of West Side Story; so ‘Danger Science’ is born, “mixing water with electricity.”

With all of these unknown criteria, one almost expects there to be a bum note, a plot point that goes awry or a missed line. But this cast is faultless from start to finish. Each character’s distinct personality comes across – romantic French chef Jacques uses of the double entendre (baguettes and buns in ovens feature a lot) and hard-faced New Yorker Lydia strikes fear into the hearts of the children. There is no stand-out performer because everyone works together to produce something quite spectacular. The songs are inventive and hilarious, and the plot flows smoothly – they even work in Class-Y Title into the dialogue as a tongue-in-cheek plotline resolution. It’s hard to imagine that this isn’t a carefully rehearsed show the company have spent months perfecting. A true class act well worthy of full marks and its place on the West End (excuse the puns, which are not quite up to the show’s standards).

Showstopper! plays at the Pleasance Courtyard (venue 33) until August 30 as part of the Edinburgh Festival Fringe before transferring to the Apollo Theatre London from September 24. For more information, visit the Fringe website or the Apollo Theatre website.