Last night's viewing: The Slap, BBC4

Do you hear a voice in your head when you read a novel? I'm willing to bet that you don't. Novels often have an authorial voice, and that voice can have a distinctive character, but it would be an odd kind of thing to personify an omniscient narrator, to decide upon its gender and age and class background.

In a television adaptation of a novel, though, you don't really have a choice. You either leave the narrator out completely or, as in The Slap, you have to cast someone to butt in now and then. And in The Slap, unfortunately, he sounds distinctly smug about the fact that he knows a lot of stuff we don't. So, as we watched Hector rousing himself from sleep, the voice explained that the winsome girl we'd just seen in soft focus was Connie: "For a moment, Hector luxuriated in memories of her, but then made his resolve 'to sort things out'." And the fact that you could hear the mocking inverted commas on that last phrase somehow only made things worse. Give the bloke a break, you thought, we've only just met.

It's a curiously old-fashioned detail in ABC Australia's adaptation of Christos Tsiolkas's best-selling novel, a knowing book, it's true, but not in my memory quite as cockily knowing as it sometimes sounds here. The good news, though, is that the voice sounds as if it might be a stopgap solution, priming the pump at the beginning of the narrative (when we really do need a bit of assistance) and increasingly fading away as the dissatisfactions of the characters are revealed by what they say to each other, rather than what's murmured in our ear. And the arena for these disclosures, as in the original novel, is Hector's 40th birthday party, a celebration that unfortunately coincides with a chronic case of midlife itch. Hector would like to scratch it with Connie, who works in his wife Aisha's veterinary clinic, but he's not nearly dim enough to be able to persuade himself that he's entitled to this excursion. Desire makes him work up excuses and guilt almost immediately cancels them out.

His day doesn't get any easier, because the family barbecue Aisha has organised tugs on other weak seams in the fabric of his life. He has to keep the peace between Aisha (Sophie Okonedo) and his Greek mother, the sort of woman who can deploy a tray of baklava as a tactical weapon. And he's struggling with his relationship with his first born, who's just a little too plump and a little too lethargic to fit his template for a perfect son. Hector looks wistfully at his cousin's boy Rocco, and then catches himself feeling wistful and feels bad about that too. When his mother makes him a present of a trip to Greece, threatening a long-planned trip to Bali with Aisha, he has yet another problem to juggle, along with Connie's seductive presence and the disruptive behaviour of Rosie and Gary's young son, Hugo, whose every tantrum and outburst is treated with indulgent fondness by his parents.

All this was very deftly done – a matter of neat choreography and furtive glances that meant that the scenes simmered with barely bitten-back antagonisms. And then the slap of the title – delivered by Hector's cousin Harry to the little brat – struck directly on the shear plane of this small group, breaking it into fragments with lethally sharp edges. Tsiolkas's original book cleverly exploited the fact that this indefensible action would provoke a defence and that all kinds of allegiances – of generation, gender and social background – would find themselves tested by it. This well-acted and faithful dramatisation preserves all that, along with the book's multi-perspective structure, with each episode focusing on one of the main characters as the narrative continues to unfold. Next week, it's the turn of Anouk, a soap-writer who knows quite a bit about hooking an audience's interest. I think she'd have approved of this as an opening episode.