James Phillips’ play, The Rubenstein Kiss, is inspired by the true story of Julius and Ethel Rosenberg, an American Jewish couple who in 1953 were executed by electric chair for allegedly providing atomic secrets to the Soviet Union.

After debuting in 2006, the play is now showing at the Southwark Playhouse, where it’s been deftly revived by director Joe Harmston.

The story spans two timelines; jumping between the Rosenbergs (here renamed Jakob and Esther Rubenstein) in the 1940s and 50s, and their son Matthew some 20 years later as he battles to clear his parents’ names.

The first act is heavy on exposition, with some rather stilted political discussions around the kitchen table. The Rubensteins, and in particular Jakob, gush with an unswerving idealism that is at best grating and at worst unconvincing. Maybe that’s partly the point but without much insight into where their convictions came from I found it hard to swallow.

Things start to pick up when the principles they hold dear come up against the inconvenient realities of life. Esther’s brother David (played excellently by Sean Rigby) and his wife Rachel are trying to start a family. When Jakob asks them to make one sacrifice too many in the name of his communist beliefs, it’s the idealist’s humanity that is called into question.

At the same time an increasingly fraught Matthew – powerfully brought to life by Dario Coates – struggles to make sense of his parents’ decision not to confess; a decision that sealed their fate as martyrs and, in turn, his as an orphan. Also working to uncover the truth is his girlfriend Anna, played by the equally compelling Katie Eldred, in her debut professional performance.

As past catches up with present the two timelines intertwine and it’s here that Harmston’s traverse set-up and intricate blocking of the actors, aided by some clever lighting and sound transitions, come into their own; working to highlight the connections between these two conflicts occurring a generation apart.

The Rubensteins’ fate of course is never in doubt. But what keeps the play alive are the questions it poses, and largely leaves unanswered, around what lies beneath the couple’s decision to sacrifice not only their own futures, but that of their son Matthew’s. How can it make sense to pursue a cause in the name of humanity when it leads us to act against the most basic human instinct to protect our own children?

It’s just a shame that Phillips’ obvious preference for the political over personal means we’re rarely given the chance to see these questions come to life through the characters grappling with them – denying us a deeper sense of the very real people who died for what they believed to be the right answers.