Before the National Theatre produced Dylan Thomas’s Under Milk Wood they had to make a decision. How could they stuff this dazzling, rapturous comic tone-poem with misery and pain? The policy at the NT is that ticket holders must endure a play rather than enjoy it. They had four options. Racism, homophobia, misogyny and mental illness are the sources of woe most favoured by modern theatre-makers. The NT duly ticked box four, mental breakdown, and hired a writer, Siân Owen, to supply the necessary dollops of torment by penning a one-act melodrama as a preamble to the script itself.

The setting is an old folks’ home which looks like a branch of Wetherspoons or an activity centre for pensioners. A real care home is full of colourless wilting old wrecks, chiefly female and mostly deaf, who do nothing all day but sit in chairs staring straight ahead of them. Activity is rare. Conversation rarer. But in this place everyone buzzes with energy and purpose. The women quarrel over the TV set and clack away at their knitting needles making scarves and tank-tops. The men, who are as numerous as the women, fill in crossword puzzles and build complicated model ships. Everyone is Welsh, including the employees. That seems wrong. Where are the Poles, the Slovaks, the Filipinos? Many care homes in Wales have difficulty recruiting local staff.

Only one of the inmates suffers from a disability. His name is Richard Jenkins, which is a joke for the cognoscenti. Richard Burton, who narrated the play for the BBC in 1954, was born with the surname Jenkins. The old chap is told that a visitor is due to see him but he doesn’t understand because dementia has stolen his memory. In bursts a fat angry man from Port Talbot (Michael Sheen), who demands to be taken to his estranged father. Some obscure historic event has prevented them from meeting earlier and the son appears to be racked with guilt and remorse. He takes it out on his poor dad, ranting at him furiously and forcing him to look at an ancient photo album. This scene is gruesome to watch but it does the trick. It makes the audience squirm in agony. Then the lights change. The torture is ended and the play starts. The idea is that the script will jog the old man’s memory and cure him of Alzheimer’s.

Thomas located his play in an imaginary seaside town, Llareggub, populated by crowds of warm and vivid personalities. On stage, the elderly thesps who played the old crocks in the care home have to impersonate much younger characters. This gives an odd flavour to some of the innuendos. ‘I don’t care if he’s common,’ says a prim old actress, ‘I want to gobble him up.’ Thomas clearly had sex on the brain when he drafted this play and he smuggled a schoolboy gag into the title. Milkwood is a species of tree that secretes a polymer used in the manufacture of condoms. The script is full of erotic hints and allusions. Organ Morgan, the chapel musician, is up all night on his ‘organ, organ, organ!’ His wife, Mrs Organ Morgan, seems perpetually tired. ‘I’m a martyr to music,’ she says with a crafty grin. The joke still works superbly.

The visuals in Lyndsey Turner’s production are very effective and Thomas’s rhapsodic, lyrical language is indestructible. Sheen plays the First Voice with all his customary wild-eyed passion. Perhaps he overdoes the fruitiness at times. This is a decent and enjoyable show despite the NT’s attempts to punish the audience for turning up.

Staircase, written by Charles Dyer in 1966, is about a persecuted gay couple living in Brixton. Charles is a fading actor who runs a barbershop with his lifelong companion, Harry. Both men accept their status as criminals and they adopt the insulting language of those who condemn them. ‘The trouble with our sort is we’re never left with anybody,’ says Harry, as if he suffered from a deformity or a disease. Charles recalls making an unwise quip when appearing in the dock. ‘In the theatre, are you?’ said a sneering judge. ‘Yes. And which panto are you in?’ He now faces a fresh charge for propositioning an undercover policeman and if he’s convicted Harry will be left on his own. Dyer’s language would have delighted Quentin Crisp. ‘We’ve got some marzipan roll from yesterday’s elevenses,’ says Harry. He dreads the thought of a geriatrics’ home: ‘Eight cubic feet for dying in.’

This is a solid rendering of a tricky and sometimes overly static drama. The design is good and the acting is executed with a brilliant and intelligent understanding of the characters’ dilemma. Social anthropologists will find the show fascinating. Theatre geeks will regard it as a must-see because it inspired Jean Poiret’s cult hit, Les Cages aux Folles. But as entertainment it can’t help but lower the spirits.