You could tie yourself in futile knots trying to decide whether this lovable series is at bottom a sitcom or a drama. Its micro-observed marital riffs can go either way. Last week the drama side broke through loud and clear as we gathered the truth about Roger and Val's buried grief, the great sadness that had been hinted at in glimpses of the old photo showing them holding a baby. This week there's more in that vein, although laced as ever with deft slapstick and dabs of the absurd. At the start we hear Roger's groans echoing through the house as Val gets in from work. Something dreadful has happened. "Is there any way back from this?" wails Roger and, by the end, it looks as if there might not be.

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.

Roger and Val: Beyond the Curtain Hooks

"It can't just be about curtain hooks. What do they slam the door against and what's already in the house that they can't escape?" This was the question posed to [co-writer] Emma and me in the first meeting with our then-potential script editor, Hugo Blick - the genius behind such works as Marion & Geoff, Up In Town and Sensitive Skin.

"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.

With Roger's dad on the verge of dying, the last thing he and Val need is a houseguest. But Val has panicked and invited cousin Cathy and her husband to stay in the spare room, so they can say their last goodbyes. Typically, on the surface, the tension here isn't in the grand drama of death, but in the fact that nobody has stayed in the spare room before. And then, quite neatly, it becomes about the bigger picture after all.

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.

Dawn French and Alfred Molina's dour "comedy" spent half an hour on a row about a fridge last week; tonight, it's about the dining room curtains - though of course, it turns into something much more meaningful than whether the fabric is hanging correctly. Despite the intimacy of the home setting and the real-time unfolding of events, it's been hard to feel connected to Roger and Val. It's all so stagey and ever-so-slightly pleased with itself.