First night: The Effect; Cottesloe Theatre, London Dramatist Lucy Prebble and director Rupert Goold certainly can't be accused of dodging difficult subject matter. They scored a huge success with Enron, which drew high praise for the vivid physicality with which it conveyed the mind-bending ingenuities of corporate fraud. Now the dream team is reunited on The Effect, Prebble's follow-up play. Searingly well-performed in Goold's co-production with Headlong and choreographed in a manner that is tinglingly alert to the fierce emotional geometries of the piece, this four-hander brings the author's agile wit, intellectual penetration and a fresh, deeply affecting empathy to bear on a fundamentally much more complex topic than finance: brain chemistry and what it can - and cannot - tell us about the causes of severe depression and the experience of being in love. All straight lines and sleek minimalism, Miriam Buether's in-the-round set converts the Cottesloe into the residential research unit of a pharmaceutical company where psychology student Connie (excellent Billie Piper) and Jonjo O'Neill's Northern Irish, attractively subversive Tristan meet as fellow test-subjects (for cash) on the trial of a new super-antidepressant. They have been deliberately chosen as non-depressives and as the dopamine kicks in, they rebel against the monitored rigidity of the procedure with a flirtation that begins with banter over urine samples and escalates, via what can be described only as a mating tap-dance, to explosive, full-blown passion. But given the chemicals with which they have been pumped, can they trust their emotions? "I can tell the difference between who I am and a side effect," Tristan declares. But Connie, whose agitated mix of wired-up emotional transparency and wariness is most movingly communicated by Piper, is sceptical of the feelings that are coursing through her. When he sees the central couple's pulsing brain scans, Tom Goodman-Hill's smug Dr Sealey is convinced that it indicates a successful antidepressant effect. For him, it's not a case of the brain mistaking what it is undergoing for love, but of love as the brain's creative rationalisation of the neural excitement. Anastasia Hille's troubled Dr James has a good reason for contradicting this, though: she knows that one of the patients is on a placebo. As Connie and Tris descend from erotic bliss to paranoid aggression, the doctors fracture into opposing extremes, with the unravelling Dr James charging her former partner of promoting the view that depression is purely a matter of chemical imbalance as a way of excusing his moral responsibility for her own breakdown. This is a provocative and challenging play and, as someone who has long had cause to be grateful for breakthroughs in medication, it ends in edgy gesture of good sense that made me feel like cheering.