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 could easily miss Stephens’ use of quantum mechanics as a way to examine the uncertainty of the human heart, despite the fact that Heisenberg is briefly mentioned. However, the theme of unpredictability features clearly, with Georgie encouraging Alex – a man with decidedly regular habits – to take a risk. And a relationship with her would certainly involve a fair amount of uncertainty. Early on, she admits she has lied to him, and her motives remain ambiguous. Why is she reaching out to him? A genuine need to connect with someone? Or a way to tap into a susceptible man’s bank balance?

Alex, meanwhile, may lead a regulated life, but he has an insight into uncertainty through music. For starters, his taste is incredibly broad from Bach to dubstep, and when he plays Georgie a Bach sonata, he asks her to predict what will happen next, saying: “It will take you completely by surprise”, and adding that the secret in music is in “the spaces between the notes”.

Heisenberg premiered in New York in 2015 then moved to Broadway the following year, while the West End production opened in 2017. Tom Healey directs the play for Melbourne Theatre Company on a makeshift, open set by designer Anna Borghesi, which sees the two actors (Kat Stewart and Peter Kowitz) moving furniture into place for different scenes, from a pile on the side of the stage. It’s a functional design rather than a staging that enhances the theme of the play.

Stewart and Kowitz are both impressive as the two mismatched characters. Georgie says that she hopes Alex will find her “exhausting but captivating” and Stewart captures her scatty, pushy, eccentric, full-frontal unpredictability with a whirlwind performance, while Kowitz portrays Alex’s gentle, accommodating constraint and his surprise at Georgie’s interest in him with beautiful understatement.

But despite two strong performances, it’s hard to really believe in the characters or their relationship. Stephens (who did the brilliant stage adaptation of Mark Haddon’s novel The Curious Incident of the Dog in the Night-Time, and whose own plays include Punk Rock and Birdland) has written an enjoyable play with Heisenberg that is certainly entertaining, but one that doesn’t entirely convince on the human front, or move you emotionally.

Heisenberg plays in the Fairfax Studio, Arts Centre Melbourne until July 3