It must have seemed well-nigh impossible for Daisy May Cooper to follow up a sleeper hit like This Country, which drew so heavily on her West Country upbringing with her brother Charlie. But she’s found fertile new territory with the new BBC comedy-drama-thriller Am I Being Unreasonable?

It shares with its forebear a similar Cotswolds location – albeit a more middle-class version – while Cooper’s character Nic has some superficial similarities to Kerry Mucklowe. She’s a little lazy – we first meet her stretched out on the sofa watching daytime telly as her son Ollie begs to be taken to school – and insular, building a cocoon of detached indifference around her to avoid addressing her real feelings.

For it’s soon revealed that she’s grieving a loss she cannot share with anyone while merely going through the motions of a moribund marriage to Dan (Dustin Demri-Burns), a decent enough bloke but someone she has no real feelings for. Nic also has no friends in the village, facing a frosty, passive-aggressive reception at the school gates every morning, which she exacerbates by her prickly personality.

That is until she bonds with a new neighbour, Jen, forming a friendship in the tried-and-tested way of slagging off shared acquaintances. And booze. They get raucously sloshed together, prompting Jen to tell tales of appalling dates and Nic to start revealing the secrets she’d never share with everyone.

But even early on, there are intriguing clues that this will not be the quintessential female friendship Nic has always craved, and that something more strange and nefarious is afoot. These tantalising hints of plot twists to come will keep you gripped, as if the excellent performances and witty dialogue in the more comic scenes were not enough.

Cooper perfectly conveys the range of emotions her character requires, broken and vulnerable when alone while outwardly tough and unfeeling in company. Her real-life bestie, Selin Hizli, both co-wrote the script and stars as Jen, who you genuinely feel like you’d want to be friends with, at least until some of her more problematic behaviour emerges. Their genuine chemistry as mates spills over the screen too.

Demri-Burns brings some tenderness and humanity to what could be a pretty bland role, but the big revelation is Lenny Rush as eight-year-old Ollie, wise beyond his years and utterly at home with the fact he’s a bit weird compared to his contemporaries. The teenage actor, who has dwarfism, is a master of the deadpan, and from his first scene it’s impossible to see how the show would have worked with anyone else cast in this key supporting role.

Am I Being Unreasonable? – the name taken from the classic question so often posed to Mumsnet’s nest of opinionated vipers – is ambitious in its ever-shifting tone, with some moments being uproarious, some heartbreaking, and some sinister. Yet it moves through the gear changes pretty smoothly, thanks to the fact you’ll be so invested in Nic and the messy, unfulfilled life she leads and so gripped by constantly surprising plot, helped along by His Dark Materials screenwriter Jack Thorne as script editor.

It’s not quite like anything else on TV…and how refreshing it is to break from formulaic formats.

• Am I Being Unreasonable? is being shown on BBC One on Friday nights. All six episodes are already available on iPlayer.