Simon Stephens’ play Heisenberg begins at London’s St Pancras Station. Georgie, a brash, motor-mouthed 42-year-old American woman, has just planted a kiss on the neck of Alex, a quiet, Irish-born, 75-year-old butcher, who was sitting on a bench.

Kat Stewart and Peter Kowitz. Photograph © Pia Johnson

They have never met before, so Alex is naturally taken aback, but Georgie isn’t letting him off the hook and just keeps talking, even while promising to leave him alone. Five days later she turns up at his butcher’s shop. Though they could hardly be more different as people, they are both lonely souls as we will discover. Over the course of the 80-minute play, we watch an unusual, somewhat improbable relationship develop.

German physicist Werner Heisenberg is famous for his uncertainty principle, after he discovered in 1927 that it is impossible to measure both the position and velocity of a particle simultaneously. Where Michael Frayn applied the uncertainty principle brilliantly to the structure of his remarkable, complex play Copenhagen, in which Heisenberg himself featured as a character, Stephens uses it subtly to underpin this rather oddball rom-com.

It’s lightly done; in fact, without the play’s title you...