I laughed out loud five times during the first three minutes of Only Child, which stars Greg McHugh as a semi-successful actor who returns to his home town to look after his ailing dad (Gregor Fisher). That’s probably the highest compliment you can pay the opening episode of a new sitcom – but it’s also the only feasible way to sell this series: there isn’t anything remotely original or different about this doggedly conventional comedy. Set in Forres, a sleepy town in the north of Scotland, the show is geriatric in pace and content; a gentle, bittersweet exploration of midlife’s handbrake turn, when children start parenting their own parents. And yet Bryce Hart’s script is meticulously dense with gags, delivered with exemplary timing by its two leads. In other words, it’s pretty funny.

We begin with McHugh’s Richard arriving home for a flying visit; he doesn’t have long before he must journey back south to film the latest series of his cosy police procedural Detective Manners (he plays Doctor Sparrow, whose imperfect grasp of basic medical principles enrages the local GP). But he doesn’t even make it out of the station before his dad is on the phone, pestering him for help with his iPad. The tech-literacy generation gap is a groan-worthy subject at this stage, but Ken’s gruff reaction to his tablet-based confusion (the iPad in question is actually the kitchen scales) sells the joke. Then we’re promptly whisked off to the funeral of a man Ken can’t stand (Richard: “Why are we here?” Ken: “Well it’s a social contract – you go to someone’s funeral, they come to yours.” Richard: “Not if they’re dead … ”).

At first, any truly worrying signs are obscured by Ken’s litany of quirky behaviours: driving around town with a spine-chilling child-sized doll named Percy in his motorbike sidecar, barking at a nuisance cat, turning the contents of his fridge into an antique collection, his passionate devotion to Jane McDonald. But when Richard witnesses Ken mixing up his pills and finds a battered car in the garage (it was driven into the local outdoor swimming pool), he realises his dad has become a danger to himself since becoming a widower a year ago. As much as he’d love to tiptoe back to London, Richard doesn’t feel great about leaving his father – so when he gets the news that Detective Manners has been cancelled (delivered by his breathtakingly brutal agent, voiced by Jennifer Saunders), he realises he has no excuse not to stick around.

As Ken, Fisher – most famous for playing Glasgow alcoholic Rab C Nesbitt on the BBC over a 28-year period – is a master at squeezing every drop of comic juice out of any given line. This is a man who could make reciting the alphabet uproarious (in fact, he does just that: upon being asked about his mental state by the GP, he reels off his ABCs – incorrectly, unfortunately). In contrast, McHugh doesn’t get to showcase the full extent of his powers – the ones in evidence in Fresh Meat (I could not have loved him more as paranoid nerd Howard) and his BBC Scotland sitcom Gary: Tank Commander, in which he played a puppyish corporal. Still, his character’s relative seriousness does invite plenty of abject humiliation, usually involving the indignities of minor celebrity (and its accompanying vanity) or Emily, the school friend he has a crush on.

After a cracking first episode – which ends with a subtly devastating moment of father-son bonding over the loss of Ken’s wife and Richard’s mum – the series does get a bit repetitive and sluggish. Its saving grace is the relatable truth at its heart: Richard’s pivot from myopic self-involvement to reluctantly but dutifully caring for his dad is a transition most of us will experience sooner or later, and the love between the pair is portrayed in a touching yet extremely realistic manner, bound up as it is in their stereotypically masculine emotional constipation.

There’s a lot to like about Only Child. It is heartwarming without being cloying and its supporting characters are great value (Paul Rattray is very amusing as Richard’s kleptomaniac childhood friend Digsy, as is Stuart Bowman as bin-fixated neighbour Rod – we also get the great Samantha Spiro as a famous actor who visits the village for an episode of Who Do You Think You Are?). Yet while Ken’s antics spark a reliable stream of comic relief, it’s impossible to imagine this sedate comedy setting the world alight.