Nova Scotia, Traverse Theatre, Edinburgh

With the emergence of Nova Scotia, John Byrne's cult Slab Boys trilogy, begun 30 years ago, officially becomes a quartet. Of the boys introduced as colour-mixers in the slab room of a Paisley carpet factory, the central two are now in their sixties, struggling to hold on to their credibility and catch the buzzwords in a new Scotland. Phil McCann (played by Paul Morrow) makes Victor Meldrew seem almost reasonable, while, with his bad leg, Spanky Farrell (Gerry Mulgrew) brings new meaning to an old hippie. There is much humour in Byrne's keenly observed satire, but below the surface there's a minefield of human fears and fragilities, and a dangerously dark narrative thread catching out the audience mid-chortle.

McCann is trying to pump some colour back into his reputation as a painter, living in a dilapidated Highland retreat with Didi (Meg Fraser), a much younger conceptual artist and a celebrity, and their two children. In a scene between McCann and a breath-takingly supercilious female BBC arts presenter (a wicked caricature captured by Cara Kelly), the interviewer dismisses him and his cronies as "a bunch of incontinent, over-the-hill tossers for whom the writing is so very clearly on the gallery wall".

Didi is having a fling with a superficial young video-maker who has arrived to film a rock star who is none other than McCann's old pal, Spanky. Vertiginous lunacy, old rivalries and cunningly engineered farce explode in a kaleidoscopic chaos while cultural attitudes and achievements get a bruising going-over. Sparring turns into fisticuffs between the two old mates, still competing for the affection of the lovely Lucille (Gerda Stevenson), who is currently remarried to Spanky.

In a wry joke about mastectomies, Spanky – it's more than his mobile that's breaking up – blubbers about being tricked into taking damaged goods, while McCann remains stonily silent on being told that Lucille was diagnosed with cancer. Frankly, why Lucille would bother with either of them, "rotten bastards" both, is hard to fathom.

Apart from potential parallels between McCann and Didi and the unorthodox relationship between Byrne and his partner (and mother of their two children), Tilda Swinton, Nova Scotia draws on poignantly personal material relating to Byrne's own origins in a family scarred by incest. The play feels too frivolous for too much of the time, too weighty for too little, and rather too long.

It is scarcely such stuff as the Slab Boys' dreams were made of yet in Paddy Cunneen's humane production, Morrow's resentful Phil, Mulgrew's comically self-obsessed Spanky and Stevenson's long-suffering Lucille, all from the original Slab Boys line-up, sparkle. It's a world into which, I hope, we haven't peeked for the last time.

'Nova Scotia' runs to 24 May (0131-228 1404)