Our New Girl, Bush Theatre, London

Fictional nannies tend to range from the magical Mary Poppins ("practically perfect in every way") to the Nanny from Hell who arrives nursing secret dreams of a twisted takeover.

Eschewing the easy, extreme ends of that spectrum, the young Irish playwright Nancy Harris handles the nanny trope with a wonderful sly, sharp humour and thematic subtlety in Our New Girl. Premiered now in Charlotte Gwinner's perfectly pitched and immaculately acted production at the Bush, the piece gives notice of a fresh, natural talent to relish.

Annie, a 28-year-old Irish farm girl (a wonderfully watchful and quirkily laconic Denise Gough) fetches up at the sleek London residence of her prospective employers only to discover that the husband, Richard, hasn't even told his wife that he has hired her. He's a dishy and unstoppably smug plastic surgeon, celebrated as the photogenic face of charitable work amongst the mutilated in war zones and disaster areas, and he's sent up here with delectably restrained hilarity by Mark Bazely. As Hazel, his beautiful, beleaguered and heavily pregnant ex-lawyer wife (excellent Kate Fleetwood) remarks: " I'm keenly aware of the fact that one us would have to lose a limb or something in this house to get you to stick around."

Ludicrously, you can't move in their kitchen for bottles of olive oil, because, on a recent holiday, Hazel developed a girl-crush on the kind of Sicilian earth mother that she thinks would like to be and offered to be her British agent. In fact, she's deluded and conflicted about her maternal feelings and seems to have got into a worrying We Need to Talk About Kevin stand-off with her hurt and rebellious eight-year-old son, Daniel (played to perfecton by Jonathan Teale on the press night).

The stage is thus set for a stingingly perceptive take on (inter alia) modern philosophies of parenthood and the sometimes farcical gap between the right-on spiel-spouting creed and the self-serving practice. The play is acute about the damage that secrets and the lies that can be inflict upon children. Some of these considerations constellate here round a pet tarantula - not a real one, because a tarantula-wrangler would have been too expensive to hire, but quite real-enough-looking (and acting) for me.

Annie does turn out to have an ulterior motive for taking the job -- one which further exposes the impermeable self-love of the husband who is a monster of true philanthropy executed as a sort of vanity project. I did not believe in the ending but if 2012 brings us many better plays, we'll be lucky indeed.