It was never going to be an easy play to watch. Stephen Unwin makes his debut as a playwright with All Our Children, a chilling expose of the brutal programme that saw Nazi Germany send thousands of disabled children to their deaths, ostensibly to ease the financial burden on the state. Over the course of one day, we see the situation through the eyes of five characters, each with a different perspective – and leave disturbed and shaken by the horrors human beings are capable of inflicting on each other.

The subject matter sounds grim, and it is – not for what we see but rather what we don’t. There are no children in the play; we never leave the comfortable office of Dr Victor Franz (Colin Tierney), chief paediatrician at a children’s clinic near Cologne. But we come to know them, through the pain of a mother who’s lost her son, the remorse of another who’s realised the patients in the clinic are, after all, “just children”, and through the cowardly attempts of a man who once swore to do no harm to justify sending his innocent charges to be murdered.

It’s this, more than anything, that really sends a chill down the spine. Franz is an experienced and compassionate doctor; he’s often funny, has an obvious affection for his devoted maid Martha (Rebecca Johnson), and dislikes the odious SS man Eric Schmidt (Edward Franklin) who’s there to make sure he toes the line and meets his grotesque quotas. Franz could be quite a likeable guy, in fact, but for the cold, clinical way he reels off the official justifications for his actions. Unlike the fanatical Schmidt, who simply hates the clinic’s patients and everything they represent, it’s obvious from the doctor’s hangdog expression, late night drinking and constant efforts to hide the truth from Martha that he knows full well what he’s doing is wrong. The arrival of David Yelland’s Bishop von Galen (a real historical figure, whose public opposition to the programme was key to its eventual abolition) could hardly be more timely, and his dignified rage in the face of Franz’s cowardice speaks for all of us.

The play is a very personal project for Stephen Unwin, who also directs, and there’s no doubting the passion or anger behind every word – but he resists the urge to preach his views, instead presenting a sensitive and balanced debate from which ultimately it’s the compassionate voices that cry out the loudest. While the men each get their turn to argue the intellectual and moral points of the debate, the two women – both mothers – represent the emotional heart of the play, and it’s their scenes that really drive home the horror of what’s happening. Lucy Speed’s Frau Pabst breaks our hearts as she describes her son with none of the eloquence of the men but a great deal more feeling; she knows Stefan will never have a job or pay tax – but he’s her son and she can’t bring herself to share the view that his is a life not worth living. And Martha’s softly spoken realisation that the patients she used to feel so sorry for are no different to her own three “normal” children has just as much impact as the bishop’s outrage.

A few slightly artificial sound effects aside, All Our Children is an incredibly effective and thought-provoking piece of theatre, a warning from history that reminds us of our continuing duty to look out for those who need our help, particularly at a time of government cuts and growing intolerance. We may not be in Nazi Germany, and it may not be 1941 – but that doesn’t mean there aren’t still lessons to be learnt, or battles to be fought.