The Uninvited (ITV) continues to show an almost touching devotion to the cliches of the science-fiction conspiracy drama. This week, for instance, Douglas Hodge's doggedly suspicious photographer found himself working inside the headquarters of Zantec, an eerily benign corporation in which all the employees wear colour-coded jump-suits and call each other "friend". The factory also has one of those computer-generated female voices that make admonitory announcements over the public address system. When she repeated the words "Refreshment Break" last night, in tones precisely balanced between invitation and threat, I was suddenly reminded of another cult series currently on our screens - the voice was virtually identical to the one that tells Tinky-Winky and LaLa that it is "Time for Tubbietoast".

This wasn't a helpful comparison. Because in any kind of plausibility face-off Teletubbies would win hands down. After all, it's not that unthinkable is it - an idyllic community of four soft toys living with a canine vacuum cleaner in a large burrow? Their world at least has an internal consistency to it - its laws may not be familiar but they are dependable once you have learnt them. In The Uninvited, on the other hand, no working assumption about the narrative rules is safe. At one moment, the baddies are so prescient that they can turn up in force just as Hodge prepares to dive to the drowned village - but 20 minutes later, he is allowed to take pictures of the incriminating site without hindrance. At one moment, the invading aliens are prepared to murder 200 villagers in order to take over their identities - but then they're sentimental enough to keep a botched body-snatch in an old people's home, where she can spill the half-baked beans to any passing snoop. Their computer systems are so powerful that they can trace unwanted access from a cyber-cafe and dispatch policemen within minutes and yet so feeble that they cannot detect a hacking attempt from inside their own headquarters. The crowning implausibility is the fact that Hodge's character is still alive - despite a reckless habit of telling the chief conspirators that he knows they're hiding something and he'll prove it in the end. Obviously these aliens hail from a planet in which a cosmic ruthlessness combines with a sense that it isn't quite fair to swat wasps, however irritatingly they buzz around you.

Some inconsistencies are apparent in Dad (BBC1) as well, Andrew Marshall's generation-gap comedy. There were two set-pieces in last night's episode that made me laugh a lot - nothing raucous, you understand, but that gentle simmering chuckle that arises from well-crafted observation. One was a passage in which Brian (George Cole) wants Alan (Kevin McNally) to ring him so that he can test his callback facility, a beautifully paced sequence of mounting frustration (the older man keeps picking up the phone, thus destroying the point of the exercise). The other was a section in which the older man sat patiently punching out labels on a Dymotape machine, infuriatingly confident that a computer wouldn't be able to do the job as well. Both were marked by their patience and understatement - there were long pauses and the laughs didn't come from self-consciously funny punchlines, but from the insinuations of the performances and the sense of recognition. To get to these moments however you had to pass through much more strident passages - where the comedy became more sketch-like and the acting - at least in Kevin McNally's case - more burlesque. Here, getting a laugh had become more important than getting it right - when McNally wanders naked into his kitchen to berate his son for making too much noise, he finds himself facing 10 or 12 teenagers. Instead of retreating or diving for cover he just stands there so that the studio audience (and us) can enjoy the naughty combination of his buttocks and those gaping faces. But there are easily enough subtle gags to make up for the ones that poke you in the ribs.