Set in a geriatric ward, naturalistically performed and filmed in a spartan, documentary style, Getting On might not sound the most inviting of comedies. Do not be misled.

Written by and starring Jo Brand, Joanna Scanlan and Vicki Pepperdine, Getting On is a gem of a show that somehow succeeds in being by turns cynical, compassionate, depressing and life-affirming. And funny. Did I mention funny?

The BBC has launched its Grey Expectations season, dedicated, as they mistily phrase it, to "the twilight years". Eighty-seven-year-old Liz Smith goes on a cruise. George Melly and John Mortimer are resurrected. And, keeping the theme alive, if that is the word, Susie Dent explains in Radio Times the meaning of the phrase "to kick the bucket". (Do not read this if you are fond of pigs.) Comfort yourself with the thought that you have the last laugh. You don't have to pay a TV licence.

The season started with Getting On (BBC4), a comedy set in a geriatric ward, which happily proved excellent. It is shot in documentary style by Peter Capaldi. All colour is leached out of the ward except a haze of institutional blue. Voices, almost ad libbing, overlap.

The patients seem set, with some spirit, on dying despite the apathetic efforts of the staff. These are Nurse Kim Wilde (Jo Brand), the lowest form of life on the ward after the lino, Sister Flixter (Joanna Scanlan), drowning in paperwork, and Dr Pippa Moore (Vicki Pepperdine), a masterpiece of tinny insincerity. Dr Moore's real passion in life is her collection of faeces ("There is a faecal deposit on that chair." "I'm on top of that"). These three wrote the script ensuring a fair supply of jokes per person. Matron is a martinet of the old school, except he is a man. And horse sense is in inverse ratio to seniority.

The first patient out of the trap is Lily, who dies on her 87th birthday as Sister Flixter is holding her hand and chuckling over her mobile. She leaves behind a large coffee cake baked by her sister, Connie. "Do you think she really wants to have her dead sister's cake back?" asks Nurse Wilde, slavering slightly. "Oh, I'm sure she does. She'll enjoy that with a cup of tea later," says Sister Flixter, fairly firmly. Connie, however, proves elusive, and they are polishing off the cake themselves when a pale, defeated face appears in the glass of the door. A Connie if ever I saw one. Sister Flixter breaks the sad news through a hail of cake crumbs, and Nurse Wilde offers a glass of water, hiding her own slice of cake behind the door jamb. It is what Lily would have wanted. Probably.

It turns out that the old Asian lady, chattering incessantly, is saying, "I want to die. Please kill me", and the nicely spoken lady with terminal MS is looking forward to a holiday in Zurich. "Oh, that's a lovely city. You'll enjoy yourself there," said Dr Moore with shining insincerity - before doing a double take and making a panic-stricken call to Dignitas.

Curiously, it reminded me of Dinnerladies, which Victoria Wood wanted shot as this is: naturalistically. It is very female and unfazed by death.

The opening line of a comedy is crucial in setting the tone - it lets you know where the writers are coming from. So when Getting On dumped "there's a s*** on the chair" on us as an introductory gambit, it was clear this wasn't going to be an easy ride. That the words were squeezed from the lugubrious gob of Jo Brand, resplendent in nurse's uniform, only piled on the agony.

Yet I can't remember the last time I howled so loudly. Cut from the same downbeat naturalistic cloth as The Royle Family and The Office, Getting On mines the misery of a hospital geriatric ward for bleak laughs. Yet for all its pot-shotting at NHS bureaucracy and patronising consultants, there's a heartening thread of humanity that stitches this mordant little gem together.

You don't need to have spent any time in geriatric wards to get Getting On but possibly it helps. Director Peter Capaldi (taking time out from political jiggery pokery in Torchwood and The Thick Of It) gets the feel of washed-out light and weary resignation spot on; even the corridors feel like they're shrugging their shoulders and doing all they can to keep from falling over.

But where Getting On really scores is with the performances of its central trio of writer/actors. Brand is matched every cynical sigh of the way by Joanna Scanlan and Vicki Pepperdine as the lap dancer-booted sister and the stool-obsessed consultant ('what type is it?' 'I'd say type four: snake') around whom Getting On revolves in increasingly desperate circles. One heartbreaking, hilarious scene summed up. An Asian woman had been muttering away in her bed for an age and finally the nurses got a translation over the phone. 'What's she saying?' 'I want to die, please kill me.' 'Put it in her notes.'

There are some real loud laughs to be had from Getting On, but they aren't comfortable, as this is a black, black comedy set in one of the more decrepit outposts of the NHS. Co-writers and stars Jo Brand, Joanna Scanlan and Vicki Pepperdine are the hopelessly incompetent staff of a pitiful geriatric ward. Brand and Scanlan are nurses rendered almost immobile by their own indolence and stupidity, while Pepperdine is a doctor who can't see her way past politically correct, coy euphemisms, as in "the deceased party" for "dead woman".

Getting On bears the fingerprints of The Thick of It, and not just because Peter Capaldi directs. It has the same ruthlessly naturalistic, documentary feel as its mighty predecessor and leaves the same lingering feeling that beneath the humour there's something very serious going on.

This is followed by Jo Brand's superb new comedy, Getting On. Set in the geriatric ward of an NHS hospital, it is centred around four brilliantly observed members of staff - a nurse newly returned to the NHS (Jo Brand), a subtly insane nursing sister (Joanna Scanlan), a male matron (Ricky Grover) and a brittle doctor (Vicki Pepperdine). Directed by Peter Capaldi, it is filmed in the verité style of The Office and The Thick of It using shaky cameras and dialogue that sounds overheard rather than scripted. It was the wonderful surprise of the week.

Here's something to savour from writer/stars Joanna Scanlan, Vicki Pepperdine, Jo Brand and director Peter Capaldi (The Thick Of It) - an extraordinarily funny, jet-black three-part sitcom set in a miserable NHS geriatric ward where the nurses are hopelessly bounded by bureaucracy and political correctness. Frighteningly familiar at times - which is surely partly down to the fact Brand used to be a nurse herself before she launched into stand-up.

You really shouldn't laugh. That's what you'll keep telling yourself during the first episode of this dazzlingly low-key new comedy set in a geriatric ward.

But it's no good putting on your politically correct face and sitting there tutting, because this is a relentlessly funny, workplace comedy that is right up there with The Thick Of It or The Office.

Part of the BBC's coyly titled Grey Expectations strand about the joys of ageing, the morbid humour is as black as death itself. Produced on a budget that would barely cover hair and make-up on Ugly Betty, it's written by Jo Brand, Joanna Scanlan and Vicki Pepperdine who also star as the self-interested, bored and incompetent medical staff. To add to its credentials, it's directed by Peter Capaldi, most famous of course as The Thick Of It's explosive spin doctor Malcolm Tucker.

Brand, as everyone knows, used to be a nurse in a psychiatric hospital which must surely account for the way that every horrible detail is so ruthlessly observed. You feel you could be watching a documentary filmed by an undercover C4 researcher with a camera hidden in a bed pan.

As the patronising, brisk, and utterly ineffectual Dr Pippa Moore (obsessed tonight with a poo that has been left on a chair), Pepperdine is absolutely spot-on and instantly recognisable, while the team's joint dealings with a patient who speaks no English are toe-curlingly sublime.

But just remember, you really shouldn't laugh.