A Slow Air, Tron Theatre, Glasgow

Atholl is a man who keeps his feelings tightly zipped inside a sturdy fleece.

Morna is his ebullient sister, as loud as her swirly tunic, as happy to have a ding-dong with some dame on the bus as she is to discuss her on-off thing with Sir Galahad (who holds court at a round table), in the local pub.

Atholl and Morna have not spoken for 14 years. In David Harrower's spare staging of his own new play, they sit on kitchen chairs, on a bare stage split by a crack down the middle. Atholl has moved to Houston, near Glasgow Airport, where his flooring business is bumping along. Morna has stayed in Edinburgh, works as a cleaner and worries about her introverted, cartoon-drawing, soon-to-be 21-year-old son, Joshua.

It is an intriguing set-up and with the skilled brother and sister Kath and Lewis Howden unfolding the story in two parallel monologues, this should be a spitting piece of drama. Harrower has a history, in his internationally successful Blackbird, of unpicking the emotional sticky stuff that becomes black and impenetrable after spending a decade in a dark cupboard.

A Slow Air promises to be equally thrilling. Joshua turns up at his estranged uncle's home, fascinated by the fact that the men who attempted to bomb Glasgow Airport in 2007 lived in the same street. He persuades Atholl to break in to their empty house for a snoop around. He walks Atholl's dog, Clay, and allows it to terrorise the daughter of Atholl's Asian neighbours. But although Morna later reveals that Joshua has a cartoon character called Crap Terrorist, the plotline fizzles out. It turns out that the siblings fell out over the usual catalogue of family grudges – money, ancient misunderstandings and whether Simple Minds are better than U2.

The Howdens play their socks off, Kath as cheeky and verbose as Lewis is monosyllabic. Perhaps being sibilings gives them the confidence to be so different; they are utterly believable, turning from hilarious dad dancing and gallus patter to real poignancy in a heartbeat. It is the drifting plot that lets them down. Is Joshua a racist ghoul? Was Morna short-changed in their parents' will? The play ends with the siblings speaking to each other. It's a shame they don't explain very much.