A Brief History Of Helen Of Troy, Soho Theatre, London <br/> The World's Biggest Diamond, Royal Court Upstairs, London

Adolescence is hard enough for any girl, but consider 15-year-old Charlotte. Her mother has just died and her father, lost in grief, makes no bones about the fact that Charlotte could never hope to emulate the dead woman's beauty. In A Brief History of Helen of Troy - Mark Schultz's punchy, perceptive and sexually explicit play - Charlotte retaliates by developing a vigorous and destructive fantasy life in which she takes as her role model the mythic femme fatale.

In a bracing exploration of the pressures on teenagers in a world where image is everything, Andrea Riseborough's knockout, high-energy performance as the gawky Charlotte is beautifully poised between hectoring, thick-skinned feistiness and fragile, thin-skinned hopefulness.

The play artfully crosses back and forth over the frontier dividing fact and wish-fulfilment, and it is not afraid to travel into some very dark places: Charlotte's vengeful response to callous jock Freddie's rebuff leads to violence of several kinds.

I could not believe in the bruising extremity of the father's rejection. But, in between, the play administers a series of horribly plausible and illuminating shocks.

On the English stage, Gregory Motton has surfaced lately more in his capacity as translator: he's adapted, for example, several works by Strindberg. The Swede's influence seems to have seeped into The World's Biggest Diamond, a two-hander in which two old lovers are reunited for a weekend after 30 years and pick at the scab of their former all-consuming passion.

Jane Asher is terribly moving as she suggests that the raw wound is still festering under her starchy surface, and Michael Feast, as the impossible invalid, pins down with a fine comic accuracy the kind of man who thinks everyone exists only in relation to him. But there is something that rings false in the extremity of the couple's rhetoric.

Patient Gareth, the husband upstairs, is only heard from once, when he flushes the loo at a moment of high drama. I was reminded of Kneehigh's Tristan & Yseult, which presented the myth from the point of view of the poor saps on the sidelines of that colossal destructive passion. By the same token, it's Gareth's version of events I'd like to hear.

Both plays to 26 November ('Helen': 0870 429 6883; 'Diamond': 020-7565 5000)