Lesley Joseph certainly hasn't chosen a comfortable, cosy return ride into town. Arriving on stage in a huge, aggressively styled hat and enormous sunglasses, she begins by snapping: "I am not angry, Felix, I am incandescent. And why are you stammering? You haven't done that since you were at prep school." Joseph was born and brought up in Northampton, but has never before appeared at the town's beautiful Royal Theatre. For her debut, she has chosen the part of abrasive Flora in Charlotte Jones's Humble Boy.

Following her celebrated role as the sex-mad Dorien in TV's Birds of a Feather, Lesley Joseph unveils a character who gets ever more loathsome as events develop. Her son Felix, who seems to be in his 30s, wouldn't dare say boo to a goose, and has never made anything much of his life - hardly surprisingly, perhaps. It would not occur to Flora for a moment that her son could be in real need of help and encouragement. Flora's husband has recently died: Felix has allegedly made his mother look a fool at the funeral, and now there's the inconvenience of disposing of the ashes. Her husband loved his bees and his garden, but now the bees have gone, and the garden is going to rack and ruin (excellently depicted in Ben Stones's set design). It's no surprise to discover that Flora has been having an affair with slimy, pretending-still-to-be-young George (Roger Sloman) for years - he homes in after the funeral like the late, lamented bees looking for pollen.

It's to the credit of playwright Jones (who also wrote the quirky Martha, Josie and the Chinese Elvis seen in Oxford last year), director Richard Beecham, and the whole cast that they keep your interest alive by contrasting the repulsive Flora and George with the several pleasant people who also inhabit Humble Boy. Besides Felix (played with expert understatement by Jeremy Swift), there's Felix's former girl friend Rosie (Amy Marston), and there's family friend Mercy (Penny Ryder), who has the thankless task of organising a truly catastrophic lunch party. A gentle ghost (Simon Molloy) looks on.

Above all, nobody forgets that Humble Boy is a bittersweet comedy, but a comedy with serious undercurrents. Charlotte Jones cites Hamlet as an inspiration, and Lesley Joseph makes it movingly apparent that Flora's attention-seeking behaviour masks grief and loneliness: she cannot actually cope on her own. By the end Flora is still snapping at Felix, but she's gone a fair way down the road to Damascus.

Humble Boy continues until Saturday, May 3. Tickets: 01604 624811 or www.royalandderngate.co.uk