For Elwood P. Dowd, the affable drunk in Mary Chase’s Harvey, the imaginary companion that brought salvation was the vast rabbit of the title. For Cillian Murphy’s Beane, the scraggy loser at the centre of John Kolvenbach’s equally funny but more serious Love Song, it’s Neve Camp-bell’s Molly, the sprite or succubus or Jungian anima who arrives in his apartment exuding all the truculence that poor passive Beane can’t let himself express or even feel.

Am I giving away one of the play’s surprises by revealing that Molly is a phantasm? Not really. After all, Beane is a loner who spends the opening scene standing in the half-light in a room that contains little but a lamp and a tacky table. It’s a bit difficult to believe that an intruder who admits to being a burglar, arsonist and “liberator”, and then proceeds to rob Beane of the spoon and cup that are his only eating implements, is a real person in a real city, even an American one.

At first the play struck me as a superior sitcom, or Neil Simon pastiche, or maybe a watered-down rerun of Albee’s Who’s Afraid of Virginia Woolf? That’s because the focus is on Beane’s sister, Kristen Johnston’s entertainingly brash, florid and Elaine Stritch-like Jo, and her wryly argumentative husband, Michael McKean’s Harry. And the audience understandably laughed a lot at Jo’s dismay at the brother who seems part Simple Simon, part monk, part screwball — and whose eccentricities include asking waiters the meaning of life while sniffing his sister’s body odour over the lunch table.

Yet Love Song is also a play about urban ennui. Lunch for Jo means taking a four-minute break from the office where she terrorises unpaid interns, firing one she doesn’t like for misfiling a single document.

And when Beane and Molly’s weird encounter escalates into a love bond they talk of crawling through syringes, vomit and city filth and plunging into pure water. There is, it seems, an alienated poet trapped somewhere inside the nonentity who says that “life is meant for other people”.

There’s a level at which Love Song is depressing, suggesting as it does that in our world happiness is to be found only in fantasy. That is emphasised not only by Beane’s transformation into a cheerful, outgoing man when his “affair” with Molly is in full flow, but by the impact he then has on Jo and Harry. The two workaholics call in sick and, aided by a bit of mime, reenact the carefree life they lack. But that would have more impact if we felt their marriage was indeed more Albee-like.

And Kolvenbach adds to the slight sense of sentimentality by failing to suggest that Molly liberates Beane’s hidden aggression as well as his thwarted capacity for joy.

Yet this is an intelligent play with a wise ending and, thanks to John Crowley’s direction, a crisp, well-acted one too. It leaves you meat, not fluff, to ponder afterwards. And that’s not often the case with American comedies today.

Box-office: 0870-060-6627