If there's a more perfect show than Spinach to take to the Edinburgh Fringe this year, I certainly haven't seen it. With its economical cast of four, intriguing premise, single set, sharp, funny writing and outstanding performances, it's a shoo-in for an evening slot at the Pleasance or Gilded Balloon.

Spinach whips along in terms of pace, too, coming in at an interval-free eighty minutes, which means you're back in the bar before 9pm: never a bad thing. The premise of the show is best expressed by the marketing material of production company Waters Edge (sic): "Imagine waking up, and you're not in your bed. Imagine waking up, realising you've been drugged and you're tied to a stranger you can't see."

This is Kate's situation when she comes round bound to a chair and to Tom, a pharmacist from Manchester who can't remember how the hell they got there any more than she can. Slowly, as the amnesia-inducing drugs wear off and their trust in one another grows, they unravel the mystery of what happened to them last night - and realise the danger they're still in. To reveal much more would spoil the plot; but rest assured it's a funny, breathless ride, and if this piques your curiosity you won't regret snapping up a ticket to find out the rest.

Out of necessity, this is a piece told mostly in flashback by two incapacitated protagonists, a situation which doesn't exactly thrum with dramatic promise; but Spinach is actually a surprisingly action-packed show. Kate (Cassandra Compton) and Tom (Ben Gerrard) swiftly slip their bonds to recount and retrace the events of the previous night, and the memory sequences are enlivened by supporting characters Darren (Craig Whittaker) and Maureen (Claire Greenway), Tom's colleagues at the pharmacy lab.

Compton and Gerrard are wonderfully cast: their slowly emerging attraction is tender and believable without losing the humour of the piece, and Compton especially (a former Eponine in Les Mis) has a sweet clear voice I could have listened to all night. Ben Gerrard oozes boy-next-door charm as Tom, and Claire Greenway shows a wide range of talents, from smoky soul singing to cello and saxophone playing. A sung-through piece like this demands a lot from its cast, and while Craig Whittaker is evidently an actor who sings, rather than an actor-singer, he brings a queasy mixture of pathos and menace to the role of Darren, and the other three are more than gifted enough to compensate.

The show's flyer promises "Thrills, abduction, pills, attraction, laughs, prescriptions ... and it's all sung!" - and it doesn't disappoint. From the minute Cassandra Compton opened her mouth to sing for help, the warm press-night audience was giggling away, and for once their enthusiastic laughter was justified: all four cast members (many of whom have West End credits) gave beautifully-pitched performances and displayed precise comic timing as well as impressive musical ability. Kevin Freeman's set is also worth mentioning for its minute detail and versatility: we move from a laboratory to a tube station to Kate's well-appointed Archway flat credibly and comfortably, and he manages to incorporate MD and composer Simon Waters's piano onstage as well.

It's hard to find fault with a show that's such a pleasure to watch, but I did have a few minor niggles which made me wonder whether writer-director Janine Waters had spent a little more time wearing her directing rather than her writing hat. (The direction, by the way, was flawless: the difficult transitions between past and present were managed seamlessly, the scenes had pace and depth, and there was never a dull moment onstage).

Firstly, I'm not sure why Spinach is sung-through: the recitative, especially at the beginning, seems to be written as dialogue and would have worked better that way. Secondly, some of the lyrics could do with a polish: given Waters's talent for snappy dialogue, it's surprising that she allowed quite so many rhymes which are obvious, lazy or imprecise to slip through. Thirdly, though the twist was ingenious and the story well thought-out, it's hard not to guess the villain early, and their descent into frothing madness seems rather abrupt. Plotwise, I can easily imagine Spinach as a Hollywood thriller - a sort of abduction-conspiracy-rom-com - but the denouement feels somewhat rushed. And finally, much of the music (perhaps because there was so much of it) didn't really grab me (though that's a matter for composer Simon Waters). Perhaps my fellow punters can't get the tunes out of their heads, but I found them a tad forgettable.

But these issues don't detract from the fact that Spinach is a truly enjoyable, well-written and very slick show: I think you'd be hard-pressed to see a better original comedy-musical this summer (yes, even if you don't like musicals) - so catch it before it sweeps the board at the Fringe.

Spinach, at King's Head, IslingtonKaty Darby reviews Spinach at the King's Head Theatre.4