"No other foodstuff is as uncompromising as a kipper," says flustered food technology teacher Val in the penultimate episode of this microscopically observed comedy. Its slow burning approach may not be to everyone's tastes, but this series about the banalities of married life has become something rather special. While not a lot physically happens (tonight Roger and Val sit on the bed, sipping tea, and stare at a computer screen), there's more going on beneath the surface. The couple (played to perfection by Dawn French and Alfred Molina) bicker about the mundane and gently bounce off one another's playful exchanges, but their lives are soaked with a strange sadness stemming from an event in their past, never mentioned until now.

In tonight's episode, Val comes home from work to find Roger hiding under the duvet in a darkened bedroom. Roger is on compassionate leave following the death of his father and has acted on the advice of his grief counsellor by writing a description of an idealised version of his life. But the nervy Roger has mistakenly emailed his heartfelt outpouring to the entire senior management team at the garden centre where he works. "Reply all - two words I'll never click again," says a hideously embarrassed Roger. "What about your job, Roger, your pension?" panics Val. As the couple try to salvage the situation, events take a more touching turn.

This quirky minimalist comedy series starring Dawn French and Alfred Molina as the Stevensons, a middle-aged couple settled in a comfortable post-work daily routine, is notable mainly for two things: first, there are no supporting actors and second, they never leave their house.

If this might seem claustrophobic and controlled it's not what French - once part of the French & Saunders duo - set out to achieve when she thought up the idea. Basically she wanted to set it in real time, with just two actors, and explore a functional, good marriage rather than the usual flawed and desperately bad one. They're hermetically sealed in, that's all.

When the couple come home from their separate jobs - she's a food technology teacher and he's a botanist working in a winter garden - they simply settle down and chat about their day. Tonight, Val finds Roger lying mute in the spare room. She tries to reach him but it dawns on them both that there's more to this crisis then the recent news about his father.

Roger reveals that, while alone in the house, he's taken up grief-counselling advice and written out an idealised self-image. So far, so ok. But then he does what most of us can only have nightmares about. He's mistakenly emailed the whole personal, heartfelt outpouring to the entire management team at his work. Hugely embarrassed, both he and Val try to salvage the situation - but will it just make matters worse?

And for the first time in the series, somebody leaves the house. Well, it has to be one of them, but which one? And why? Oh, the tension...

Roger (Alfred Molina) can't do anything quietly, and that includes coping with the loss of his father - while on compassionate leave he ends up in a tizzy when he writes an e-mail to a work colleague CC'ing the wrong people. ('I'm white-water rafting my way out of my job,' he moans). But deep-rooted grief for their late son is at the heart of this, and it surfaces in yet another beautifully crafted episode.

"Subtle" doesn't always mean "easy", as Roger & Val Have Just Got In (BBC2) keeps proving. Reviewers, like viewers, love to have easy terms on hand but this show can't be labelled as a comedy any more than it deserves to be called a drama.

Like a version of Terry And June scripted by Philip Larkin, it's proof that little is needed to make top-drawer telly, beyond a couple of rooms, a pair of great actors and a cracking script.

This week, long-married Roger and Val were doing nothing more remarkable than preparing their spare room for two unwelcome guests while also preparing themselves for the death of Roger's aged father. Emotionally and physically, this took them into desolate territory because the room in question had clearly been a nursery for a child who never lived.

Along the way, Dawn French and Alfred Molina provided plenty of smiles if not laughs but the closing shot, of a forlorn little lampshade decorated with stars and rockets, produced more damp eyes than a decade of EastEnders.

Followers of this subtle sitcom will know by now that when Roger (Alfred Molina) asks his wife Val (Dawn French) whether she has any news, the answer will be stunningly mundane. "I was on yard duty," the teacher ventures. "But I actually planned for it. I had my coat and whistle ready." This fourth episode may feature the habitual dearth of anything actually happening, but the emotional pitch intensifies as Roger's father's health declines ("He's like a death boomerang," observes Val). Adding humour to the mix is the imminent arrival of a pair of unwanted guests whose visit forces the couple to tidy their spare room, with seething reluctance.

This observant, understated series of mildly comedic mini-plays (really, it's not a sitcom) about a contentedly married couple slaps us with a half-hour that prompts more tears than chuckles. Roger's cousin Cathy and her unpleasant husband Bob (a misogynist control freak who likes to order for his dinner companions in restaurants) are coming to stay. So most of the talking happens in the spartan spare bedroom, which, we learn, no one has slept in for 17 years. As the couple strip down the mattress, faff over the arrangement of knick-knacks and work out emergency escape routes (this episode's only hoot-worthy moment), the atmosphere is claustrophobic and fraught. At first it's not clear why. When the reason emerges, the pair's marital back-and-forth, which usually veers from sanguine rumination to fidgety bickering, dissolves into drowning sadness. It's a curious move for a supposed comedy, but somehow not out of place. Four episodes in, Dawn French and Alfred Molina are still tickling us - although sometimes it's with the tiniest feather. There are never any belly laughs, but we don't need them. There's something wonderfully subtle and assured about the dialogue, like a grown-up Wallace & Gromit that's been given a polish by Alan Ayckbourn.

In Roger and Val Have Just Got In, Val came back from work with three noodle bakes, two more than she could fit into her refrigerator, which is what passes for a plot highlight in Beth and Emma Kilcoyne's daringly understated comedy. It's something of a noodle bake itself, this series: looping strands of domestic wittering and bickering in a sauce of beautifully cooked blandness, not exactly a showy dish, but reassuring and comforting in its ordinariness. I could quote lines at you all day without being able to make a convincing case for it, because it's all about context and the recognisability of the moment. As Roger and Val, Alfred Molina and Dawn French underplay it beautifully, commiserating with each other about the day's minor setbacks (11 dead rice plants for Roger, who works as a botanist at the Winter Gardens, and feels about exotic flora as Martin Clunes does about Chester) and never talking about the big drama in their life - the death of a child, the illness of a parent - things that you glimpse as if out of the corner of your eye in passing. The programme takes half an hour to go nowhere, but it's those unmentioned griefs that make it work. It's just how life goes on, quite substantial portions of it sustained by gentle self-deception and the magnification of stuff that doesn't really matter.

This sitcom, with Dawn French and Hollywood's Alfred Molina playing a likeably slothful middle-aged couple, stretches The Royle Family's concept of "comedy of the mundane" to the point where it becomes almost excruciating to watch. French and Molina are brilliant and the script is believable - this week they spend several minutes discussing whether to eat a scotch pancake - but that's the problem. Nothing funny or even slightly remarkable happens. It makes Waiting for Godot look like Bad Boys II.

This week's chore occupying our heroes involves re-hanging the dining-room curtains. That leads into meditations on the place of curtains in human evolution, whether the light makes Roger look Norwegian, and the possibility that a bag of curtain hooks could be mistaken for a bag of noodles. It's tenuous, micro-observed drama but, as played to perfection by Dawn French and Alfred Molina, it's sweet, funny and at times oddly moving. There's a never-mentioned sadness in the couple's shared past and the fact that Roger's father is dying ("He won't see his tomatoes, Val") is stirring it up. But mostly, we observe the playful, trivial exchanges of a comfortably married couple.

Roger and Val Have Just Got In is set in real time, covering the half hour after a comfortably off, middle aged couple have returned home from work. Which has to be the least enticing sitcom premise ever, so maximum respect to whoever pitched that at the BBC Comedy Department.

But dismiss the show at your peril, as it stars Alfred Molina and Dawn French in the title roles, performing exquisitely observed and intricately constructed scripts by Emma and Beth Kilcoyne.

Fish fingers in the wrong fridge, a lost receipt for a vacuum cleaner and badly hanging curtains have so far provided Roger and Val with cause for conflict, reconciliation and yet more conflict.

It's engaging, rather than enthralling, provoking smiles of recognition rather than howls of amusement.