All unhappy families, wrote Tolstoy, are unhappy in their own ways. That's not so. Think of all those families awoken at dawn by police wanting to confiscate their patriarchs' child-porn-stuffed computers. They must all be unhappy in roughly the same way: the destruction of the man's character, his wife's suspicion that he may no longer be trusted with their children, the secrecy, the shame. Kate Gabriel's Fiona's Story made sure to begin by saying what followed was based on many real-life events.

Appropriately perhaps, given its subject, it was a lugubriously voyeuristic drama, one that proved that bad things happen to rich people. The immersion in this well-heeled family's gloom was so great that we barely caught a glimpse of sunlight in the first 30 minutes, and it was a dim and wintery light when it came. The story's only redeeming feature was that our heroine, Fiona, the wife, ended up smiling - possibly at the prospect of renewing her affair with her poor but honest choirmaster, possibly at having got rid of her pervy spouse or, possibly, because she at least had come to terms with the truth.

Played with ill-complexioned fortitude by Gina McKee, Fiona was on to it from the start, never doubting that Simon had been viewing the vilest stuff. His confession to her, 17 minutes in, deprived us, thank goodness, of the twist in the tail that lesser dramas would have thought necessary. Fiona's only failure was not to follow the truth's consequences early enough. She lied to the social services, kept his arrest from her friends and promised to stay with him until the bitter end. Only when her loyalty failed to stretch to the resumption of conjugal relations did he realise that the game was up. Off he went and found a 26-year-old version of Fiona who considered his arrest “no big deal”.

Fiona's nightmare was that others seemed to share this novel opinion: his mother, who reminded her that many men had mid-life crises, and his brother (Nicholas Farrell, managing again to be simultaneously bluff and slimy), who considered Simon to be the victim of a Salem-style witch-hunt. As someone who has lazily argued in much this way, I found Fiona's quiet insistence that Simon had conspired in the rape of babies to be powerful.

In a rare instance of a TV drama sticking up for tabloid hysteria, Fiona was the sane person in a mad world. As Simon, Jeremy Northam was excellent, veering from suicidal self-pity, through self-delusion to self-satisfaction. When the police dropped the case, he lost ten years from his face. But we remembered that babyish gurgling noise he made when he thought he was heading for prison. Yet, for all Simon was revolting, it was his story, not Fiona's, we might have followed with greater profit.

Abdul Baset Ali al-Megrahi is still in jail for the 1988 Lockerbie bombing, although he has an appeal pending. The next time television makes a documentary about Lockerbie, it had better come up with a confession, which, as one of the victims' fathers said, was about the only way this matter will be concluded. Getting Colonel Gaddafi's son to chuckle that the Libyans were innocent did not cut it as a pay off. The Conspiracy Files: Lockerbie, though flashy, kept hurtling down blind alleys - much like the official investigation itself - and was unsatisfying.

Editor's choice

• U2 at Nou Camp Stadium, Barcelona
• The 50 Best Movie Villains
• The 15 golden rules of theatre etiquette