For the uninitiated: Shit-Faced Shakespeare – and now its progeny, Shit-Faced Showtime – features one drunk performer each night. The rest of the cast attempt to carry on as normal, while the inebriated one struggles, often hilariously, to remember their part.

Shakespeare was fresh and fantastic in its first year, but the formula's felt a little more tired as each year passes. Showtime – which swaps A Midsummer Night's Dream and Macbeth for a generic musical – feels like a step too far. Sure, there is evident talent among the cast of five, but the verve and spontaneity of Shit-Faced's early years is lost here.

Part of the problem with Showtime is its utter lack of story – which, in fairness, the cast readily admit in the opening number. Yes there's a basic plot about two warring parents moving to America to escape their debts, and their daughter running off with a sweet young man who turns out to run a seedy Cabaret-style club. But in the total absence of a meaningful narrative or characters worth investing in, nothing is at stake when things, inevitably, go wrong. With Shakespeare, there's a general awareness of what won't happen in the play if the drunk performer messes up, and that's funny in itself. In Showtime, drunkenness is the only joke on offer.

Still, there's a lovely, supportive spirit among the cast; the compere Dylan, who plays piano with expert speed on stage, is on constant lookout for tonight's drunk performer, who's consumed a Bulmers and a third of a bottle of vodka before the show (but is much more drunk than this might suggest). Yet it's telling that this evening's funniest moment comes from a sober performer ad libbing. Perhaps with better base material, Showtime might in time gain that frisson of unpredictability that Shakespeare has in its finer moments.

Underbelly Med Quad, 0844 545 8252, until 31 Aug (not 17), 6.45pm, £11–£12 (£9–£10).