Marjorie's quiet evening in, dancing by herself in the front room, is suddenly invaded, first by a wasp sting - and then by the arrival of a stranger who bursts in claiming he is looking for someone called Joe. But as the scene quickly escalates, it turns out he's a would-be rapist, and as his nasty verbal and physical attack is played out just inches from where we're sitting, it's very difficult to avoid looking - but you don't want to watch it, either.
There's an immediacy and sheer horror to this scene that Glenn Fraser's fierce and finely played production catches gruesomely well. And then, as the victim fights back and the hunter turns hunted, the tensions escalate dramatically further. As Marjorie trusses the attacker up and hurls him into the fireplace, throws boiling water over him and threatens to set him on fire, an intricate psychological revenge thriller starts to evolve about the politics of taking the law into your hands when you know that the law won't provide justice.
So far, so good - or rather, so grim. But where can it go? Mastrosimone, fortunately, keeps the action simmering tensely along with the arrival home of Marjorie's two flatmates. As their hostage starts to play them off against each other, a powerful argument is played out about what to do with him.
It's clearly no use for Darren Day - the musical star who was seen in the first run of "I'm a Celebrity ... Get Me Out of Here" - to cry out "I'm a Rapist ... Get Me Out Of Here". And it's amazing to see this swaggeringly confident actor reduced to spending most of the evening blindfolded and tied up by electrical cords, huddled in a fireplace. With his raspy Bob Hoskins cockney voice, Day is genuinely effective at conveying his character's ability to manipulate the women he may have intended far greater harm to.
As the victim who fights him off and then turns aggressor, Ineke Rapp is remarkable, charting a steely fight for survival that has to walk a fine tight-rope between maintaining our revulsion for her aggressor and now her aggressiveness. As her flatmates, Abby Simpson has a difficult task, too, to register a kind of blank passivity of a bystander who'd rather not be involved, while Susie Porter, trying to act as the mediator of reasonableness, has to overcome some awkward therapy-speak lines.
Extremities is undoubtedly a gruelling evening, but a brutally powerful one.