In its initial scenes, Simon Stephens’ Heisenberg presents itself as the simple story of a May December romance, but as it develops it becomes a far more interesting observation of the complexity and painful truth of loneliness. The title comes from German physicist Werner Heisenberg who developed the ‘Uncertainty Principle’ when studying subatomic particles. Expressed in the simplest of terms, this principle determines that you can either know where you are or where you are going, but not both at the same time. It’s a neat expression of how Stephens’ two characters, or particles, make the measure of each other.

Opening in a London Underground station where 42 year-old Georgie Burns (Kat Stewart), an expat American, has just kissed the neck of a stranger, 75 year-old butcher and widower Alex Priest (Peter Kowitz). The conversation that ensues entwines the couple and an intriguing relationship develops that constantly variates in unexpected directions.

Georgie is nothing less than effusive in her first interactions with Alex, someone for whom the term verbal diarrhoea might have been invented, and Kat Stewart embodies the woman’s wildly gushing nature with tantalising volatility. It’s an electric performance that energises director Tom Healey’s production and makes this 90-minute story of conversations between two people completely engrossing from start to finish.

It feels like Georgie coquettishly assaults Alex with ever-adaptive charms. The poor man seems not to stand a chance in the face of her onslaught, but perhaps he doesn’t wish to. Peter Kowitz’s Alex is a quiet man, methodical in his routine and enjoying the simple yet melancholy pleasures of his twilight years. Irish born, but having spent the majority of his life living in the UK, Kowitz places Alex’s compound accent in a way that makes it difficult to locate the story at first, but his wonderfully split expression of a man who has received an unexpected joy in a way that only highlights what he has lost is heartbreakingly presented.

Those who saw MTC’s productions of Birdland and the adaption of The Curious Incident of the Dog in the Night-Time will know playwright Simon Stephens is never one to shy away from difficult and complex characters. Likewise, the hidden depths of Heisenberg’s characters resonate long after the show is over. The simple truths (and lies) of these two characters make for fascinating analysis and no doubt certain reward upon repeat viewing as the nuanced details of the script repay additional observation.

Certainly, the focus of this production is rightly on the two characters and the way they push and pull at each other. Anna Borghesi’s set design creates false walls and exits to give the impression of an ‘empty’ stage with just a few props and furniture pieces (some looking like stage rigging) to create the settings. Bronwyn Pringle’s lighting design is atmospheric yet focused on where Healey has directed his performers to play in each scene. Compositions and Sound by Clemence Williams are unobtrusive, yet effective, as are Borghesi’s low-key costume designs.

It’s Georgie who sets this story in motion with that unexpected kiss that she can’t explain the reason for, which of course she can, and it’s in the way Kat Stewart allows us to discover those reasons that make her performance so enthralling. It’s like a masterclass in acting and reacting. Stewart’s Georgie is constantly looking to see what affect her words are having on Alex and it’s in those looks that we discover deliciously enigmatic detail.

There’s a lot left up to the audience to decide in Simon Stephens’ script and that makes this intriguing story even more engaging.