It concerns Jerome, a composer, who develops a plan to persuade his estranged wife Corinna that his home life is sufficiently stable for her to allow their daughter to stay with him.

The cast in Scarborough and London included Ian McKellen (who was knighted in 1990, becoming Sir Ian McKellen) as Jerome, Jane Asher as Corinna, Emma Chambers as Geain aged 13, Serena Evans as Zoe, Robin Herford as Lupus, Suzy Newman as Rita, and Michael Simkins as Mervyn.

It is a bleak future where law and order has broken down in at least some parts of the city, and social interaction is limited by the automation of many shops and services.

Before the play begins, Jerome has arranged a meeting with wife Corinna and Mervyn Bickerdyke from the Department of Child Wellbeing, to convince them that it will be appropriate for Geain to stay with him for long visits.

To help to convince them, Jerome plans to hire an actress, Zoe Mill, from an escort agency, to pose as his fiancée.

Much of the play's humour originates from her idiosyncrasies as she collides with the furniture, blurts out unexpected remarks, and performs her tasks incompetently – such as serving a drink with the tumbler upside down.

Zoe's arrival establishes the hostile nature of the neighbourhood, as she has been mugged by a local gang (the "Daughters of Darkness") – led by Rita.

Act 1 scene 2 begins the next morning with Jerome working on two short compositions using secretly sampled sounds of Zoe.

Because of her masculine hair and clothing Jerome initially mistakes her for a local hooligan, and she enters looking surly, unwashed and very unfeminine – in complete contrast to the delightful 9-year-old girl shown on screen in Act 1.

When he recovers Mervyn, Corinna and Geain prepare to leave hurriedly as they have had a call to tell them that their car is under siege by the Daughters of Darkness.

Corinna offers Jerome a final chance to leave all his equipment behind and come to live with her and Geain, because they still love him.

He quickly returns to the flat and replays parts of the speech, isolating and manipulating one phrase which is indeed "the sound" which he has been waiting four years to hear.

He feeds the sound sample into his complex equipment, and for the last three minutes of the play we hear the final piece as he creates it.

While this takes place, he remains oblivious to the video screen on which the audience sees Mervyn, Corinna and Geain hammering at the door in a vain attempt to persuade him to come with them.

Jerome completes his composition and is briefly ecstatic, but the sound of violence outside brings him back to full awareness.

The Guardian's Michael Billington listed Henceforward... as one of the five greatest dystopian dramas and wrote that the play works because "Ayckbourn himself is both a techno-junkie and intelligent enough to see the dangers of a world in which we start to prefer rational machines to to [sic] irrational human beings.