Heavenly, Frantic Assembly at the Soho Theatre, London

Morecambe and Wise's breakfast sketch meets shambolic child's play

It's a cross between the Morecambe and Wise breakfast sketch – physical jerks tightly and wittily syncopated by music – and the shows my children insist on performing for us in their bedroom – endlessly rehearsed but somehow immediately shambolic as soon as the audience (Mum, Dad and occasional Granny) take their seats.

This is physical theatre. This is Frantic Assembly's mission. But Heavenly, their new devised show, truly takes angelic wing when the three performers/devisers/directors are talking – sitting, standing and just talking. At its best, it's three blokes, suddenly finding themselves naked but for towelling robes and Calvins on a gently sloping incline of mattresses and Ikea sofas. This makes an ideal playmat for when they enter the "physical" interludes, rolling over each like slow-mo maggots, but I can't quite see what these high-energy ballets added (though Nathaniel Reed's score is outstanding).

From the start, they half-know and half-don't know where they are. Once they establish it is the/an afterworld, escape is their focus. First the regrets ("I was going to start flossing") and then the bargaining, the pleas, the competitiveness. They make their pleas for the wonderful world they would create if only given a second chance. Scott will adopt a homeless ugly orphan ("from somewhere grim like Romania or Cardiff") who will become the First Homeless Ugly Orphan Prime Minister. Liam will create world peace – reconciling "even Ginger and Posh". Steven will invent a time machine and zap through aeons of history, zapping embryonic psychos until he himself is fit to be zapped.

Although Steven and Liam accuse Scott of not really trying, of being glad he's there, it doesn't actually work for any of them. But then it's never going to – because real life has already decided. And this is what shifts this show on from mere blokeish backchat to something genuinely moving. The three life-long buddies (two brothers and the friend they met in the park 20 years ago) were taking a drunken, hogmanay walk along the cliff path near Aberystwyth. First Liam slips down a gully on to a ledge and then Steve goes to his rescue and then together they plunge downwards. Leaving Scott forced to choose, to die with or live. He's the first to admit he is "no good at the whole world thing" but that's no excuse.

Without giving too much away, this is how the piece derives its final punch, its ultimate worthwhileness. Until then, it is very funny (in a silly, backchatty kind of way) and sporadically sentimental – especially the Snow White and Parma Violet story of how they came to be in Aber. But it's own sharp wit is always on hand to puncture any genuine chance of true sentimentality.

And at the end, the jokes recede, the story takes precedence, the "physicality" becomes limited and meaningful, and it has all been worthwhile.