The Effect, National Theatre, London

When two people fall in love we often talk about the chemistry between them. But what is that chemistry? In part, it is this that Lucy Prebble’s scintillating new play explores. The Effect is a love story for the neuroscience age, but also a fascinating and often daring journey into the way the mind works, into our limited understanding of the very thing that governs all we do, and into the reasons for and treatment of depression. It is a big change from Enron, which shot Prebble to fame, but it demonstrates the same vivid, provocative intelligence and is directed with verve and immense sympathy by Rupert Goold in a joint National Theatre-Headlong production.

Astutely, Prebble acknowledges the fact that her characters are guinea pigs by making them guinea pigs. Connie and Tristan are volunteers on a drug trial for a pharmaceutical company, examining the effects of a new anti-depressant. Connie is a sensible, sceptical psychology student; Tristan is an impulsive, mischievous drifter. Billie Piper and Jonjo O’Neill make them funny, spiky and vulnerable and their love affair completely believable. The two tread a drolly unconventional path to romance: they meet in the anodyne surroundings of a clinic (expertly recreated in Miriam Buether’s set), notice each other over phials of urine, and fall in love during a late-night tryst at a nearby abandoned asylum.

But are they in love or are their feelings prompted by the drug? If the latter, does it matter? And what difference does it make if one of them is on a placebo? These questions trouble Connie, who, in Piper’s beautifully calibrated performance, struggles to understand her feelings for the Puck-like Tristan. They also trouble Anastasia Hille’s Dr James, the doctor conducting the experiment. Her misgivings give rise to a couple of highly charged exchanges with her superior (Tom Goodman-Hill), thrashing out the arguments for and against medication for depression. We soon realise that neither of them is objective.

So Prebble interweaves instinct and intellect, science and art, mind and body. The play has echoes of Shakespeare – Romeo and Juliet, Hamlet and A Midsummer Night’s Dream. And, as with Nick Payne’s Constellations, Prebble’s play asks what cutting-edge scientific theories mean for free will, human fallibility and the impulse to love.

The oppositions in the play sometimes feel conveniently neat and it has the odd implausibility – but perhaps that is apt for a drama that deliberately ties itself in knots over cause and effect. There are more questions than answers here, and rightly so. But what is most remarkable is how moving it is and how tenderly it is delivered. The four superb performances draw us towards a healing ending in which, whatever causes love, we see its effects.

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