The unmapped universe was the canvas for Nick Payne's West End hit Constellations, in which the same relationship played out in countless alternate worlds.
For his latest work, Incognito, the playwright focuses in on a bundle of nerve cells about the size of a melon: the human brain.
It may be physically smaller, but it holds just as many mysteries and the exploration of our grey matter throws up even more challenging questions about our own existence.
In an illuminating 90 minutes, a strong ensemble cast, including the impressive Amelia Lowdell , take us on a whistlestop tour of our neurological nether-regions that is both funny and devastating.
Payne takes as his jumping-off point two of science's stranger episodes: the 'theft' of Einstein's brain by a pathologist obsessed with uncovering its secrets, and a pioneering operation which cured a man's epilepsy but shattered his short-term memory.
He interweaves these tales with the modern-day story of a neuropsychologist struggling to balance her work and social life as she embarks on a new relationship following her divorce.
As in Constellations, Payne displays the skills of a magpie curator, building up small, seemingly innocuous scraps of lives into a whole which is much more than its parts.
A 10-minute piano recital, for example, is imbued with all the possibilities and ruinous regrets of a life half lived, while a crusading career crumbles agonisingly under the blurry gaze of a couple of stoners.
But while Constellations, for all its bells and whistles, was in many ways a fairly conventional romcom, this is far more challenging stuff, sending the ever-advancing waves of science crashing against the foundations of our existence.
In doing so, Payne questions both what we really know about ourselves and how much we want to know. When one protagonist muses about how the brain simply 'builds a narrative to steady us from moment to moment', it's not clear whether we should be horrified or grateful.
For all the questions it raises, this is a visceral and at times heartbreaking production, which, in director Joe Murphy's capable hands, keeps you hooked throughout.
It's also a play well suited to the more intimate environs of the Bush Theatre, so I'd recommend trying to catch it here rather than waiting for a West End transfer.
* Incognito is at the Bush Theatre, in Uxbridge Road, Shepherds Bush, until June 21. For tickets, visit www.bushtheatre.co.uk or call the box office on 020 8743 5050.
