Adrenalin... Heart, Bush Theatre, London

Julia Ford and Mark Monero make a wonderfully winning double act in Adrenalin... Heart, Georgia Fitch's assured new two-hander about a couple whose misalliance drags them down into the depths of drug-abusing defeat.

Leigh is a white, Catholic single mother with two kids and a job in local government. She hasn't been touched by a man in seven years, but has some "top conversations" with God. Angel is black, a charmer and a graduate of prison and university, with no proper job and a sideline in Islington drug-dealing. They meet at a "Happy Monday", to which Leigh has been dragged by Naughty Nancy, an office temp and joker, and pretty soon Leigh is having three baths and buying five sets of new underwear before she goes out, in the hope of getting off with Angel.

The play begins with briskly intercut competitive monologues as the couple irritably interrupt each other and overlap. It's as though the affair is being presented as a re-enactment with the performers cast as both participants and narrators whose accounts shift between using the first and the third person.

This form gives the proceedings the sad sense of being the chronicle of a defeat foretold – a scenario that will eventually dissolve, for all its transitional intensity – while the discrepant perspectives powerfully bring home the inequality in a relationship where Leigh, who starts off as a clean-living mother, has so much more to lose.

Erotic infatuation leads, in her, to addictions a great deal more dangerous. I was never clear about how and why Leigh's addictive nature – her hunger for "the total all-embracing experience" – had managed to take a seven-year sabbatical prior to her encounter with Angel. Perhaps we are to assume that she overdosed on motherhood ("I give/ gave everything to being a mum," she says, with one of those little changes of tense that twist the heart).

But, once she's hooked up with her lover, there's a terrible inevitability to her slide into narcotic dependence. He is her substance of choice. She isn't his – quite. Drugs blot out this imbalance, while also turning her into the kind of neglectful mother she had always despised. "I'd eat glass for you," she tells Angel. She's not exaggerating.

The marvellous and under-appreciated Julia Ford, who has never struck a false note in any of the many roles I have seen her play, excels herself as Leigh. It's not a showy performance, just understatedly true in every detail. She begins as a twinkly-eyed, engaging young woman, who is trying to conquer her shyness and her stoical resignation to celibacy. She ends up a dead-eyed wraith.

The disintegration is all the more harrowing for being so unmelodramatically acted. Mark Monero vividly communicates Angel's sexual cockiness and the shrewdness that makes him always keep something of himself in reserve.

The couple report on the sordid world around them in jolting flicks of detail – there's the 13-year-old girl Leigh meets in a drug-racketeering King's Cross brothel, who has just had a termination in return for the mobile phone her mother offered her as a bribe, or Leigh's friend who "did pills at the weekend and sucked off a Croatian in Oxford Street". But the play, trapped in two heads, can only obliquely address the social forces that create these conditions. It's heartfelt, humorous and a bit hermetic. Fitch has the talent, though, and I look forward to seeing her work on a broader canvas.

To 9 Nov (020-8610 4224)