The Weekend's TV: Fiona's Story, Sun, BBC1<br />The Conspiracy Files: Lockerbie, Sun, BBC2

Like the policemen who showed up at 6am to shatter Fiona's life, Fiona's Story held its credentials to the glass before entering. "The following is a work of fiction inspired by many true stories," read the opening title card. In other words, any complaints should be referred to real life. Which raises a philosophical question, really. If you collage a number of small truths together, do you end up with one big truth, or something that's not quite true to any particular experience? More to the point, is television drama in the truth business at all, or is plausibility more important? Documentaries can afford to ignore plausibility, because their assertion is simple: believe it or not, this is what happened. Dramas cannot, because there's nothing to fall back on if belief wavers. They have to arrive at some working compromise between what an audience thinks should happen in a particular set of circumstances and what perhaps actually did.

The policemen turned up at Fiona's door because her husband, Simon – middle-class, charming, Jeremy Northam, for heaven's sake – had been downloading child porn on the family computer. In an instant, Fiona's home – the study where her husband worked late, her daughters' bedrooms – had become a potential crime scene. The numbed enormity of this moment was beautifully done, Gina McKee dazed by what had happened and forced to juggle the banal normalities of breakfast and the school run with a house full of police officers. He insisted it was a case of stolen identity and she, blank and white-faced, tried to get through the day. But after a length of time that the play's chronology left unclear, he broke down and admitted that the charge was justified. He did do it. More than once and for a long period. Suddenly, the unhappinesses of Fiona's marriage – the death of her sex life – crystallised around this unabsorbable fact.

And if there was a problem with Kate Gabriel's underplayed, reined-in drama, it lay in the fact that the questions didn't come nearly fast enough. "Why?" asked Fiona eventually, but only after weeks or possibly months had passed, and she'd lied to social services in order to keep the family together, at her husband's instigation. It's hard not to feel that this would be the very first question a wife would ask, or that it wouldn't take anything like as long as it did here for the rage and disgust to follow it. "I didn't want you to get angry," said Simon, explaining why he had told her a development in the case in front of their children, but there had been no sign up to this point that anger was even in Fiona's emotional repertoire. When she learnt that he was still taking baths with his daughters, her reaction was tentative and apologetic, not the flare of incredulity that you might expect from a protective mother.

The oddities of Simon's behaviour made more sense: the queasy way in which, as the threat of prosecution and exposure receded, he reconfigured his actions as a containable aberration, blown out of proportion by social hysteria. He was encouraged in this by the equivocations of family members, by a mother who simply wanted some kind of normality restored and by a brother who muttered that it's a "victimless crime" (oblivious to the damage done at both ends of the internet connection). So from suicidal remorse, Simon slid, by entirely plausible degrees, into thinking of himself as the victim of his wife's unreasonable suspicions. "Am I really such a monster?" he shouted self-pityingly, after she'd questioned his feelings towards their daughters. The tragedy for her was that she couldn't confidently answer that question any more, and if Fiona's Story left you questioning the persuasiveness of her immediate reactions, it powerfully conveyed the devastation such a discovery would leave behind it.

Truth and lies were hopelessly tangled up in The Conspiracy Files, too, a strand that has done some useful work in the past in wiping up the mess left behind by credulous paranoiacs. In the case of this film about the bombing of Pan Am 103, which came down on the town of Lockerbie, matters were less straightforward. The programme briskly sorted out the facts behind some conspiracy theories – such as the nature of an early warning phoned in to the US embassy in Helsinki – while making it clear that there are still unsettling holes in the prosecution case. At least two of the interviewees struck me as bare-faced liars, while as many again had powerful motives for not telling the truth about an event inextricably entangled with the brutal realpolitik of the time. And the film ended with an astonishing interview in which the son of Colonel Gaddafi attacked the relatives of the dead as "greedy" and effectively repudiated Libya's formal acceptance of responsibility for the bombing. "I admit that we play with words," he said insouciantly. "We had to... there was no other solution."