Confusing, but in an unusual way. We can see what's happening. There's no ambiguity about that. However even after the closing credits it's still not clear what was causing it, while even the emotionally inadequate performances are likely to have been deliberate.
Their pre-credits sequence isn't the best. A boy (Christopher Reilly) goes into a laboratory that contains lots of animal cages, takes a bottle of an unknown chemical substance off a shelf and eats its contents. Thirty seconds of poor acting later, he dies. Audience verdict: "what a twat". There's also a confusing shot of what might be a dead rabbit which, thinking back, might be the biggest single plot hole in the episode. Why's the bunny dead, then?
The rest of the episode is about his replacement. Reilly's not noticeably hard-grieving parents, Barbara Kellerman and Gary Bond, presumably don't have the spare time to conceive another child in the usual fashion, so instead they adopt. Of course they had to do interviews and so on, but even so I'm sure they were delighted with the time saving. I wouldn't actually call them callous, but they're capable of being breezy. Bond is downright cheerful. He hardly seems to have a care, outside his work breeding protein-rich plants to feed the Third World. Undeniably this could be a world-changing scientific breakthrough, but even so you'd expect him to be taking more notice of the fact that his son died before his eyes and that they're about to adopt another one. As for Kellerman, she's a yummy mummy with a sometimes brittle manner and a busy life of her own. She puts far more effort into their new son than does her husband, but even so her performance doesn't begin to convey the required emotional weight.
As for the new lad, he's played by one Matthew Blakstad. He's also surprisingly old for a boy who's up for adoption, but I suppose that's just a function of TV production. It's a large role and if you must use a child actor, it's surely better to use one who's fifteen than who's six. Anyway, Blakstad's practically a robot. That's not just bad acting, since it's referenced in the script. He talks like a narrator, seems to have lost his emotion chip and calls his new parents Mr and Mrs Morton instead of "mummy" and "daddy".
This creates unusual tensions. You don't know how much of this is deliberate and how much is bad acting. Blakstad's clearly being directed to play the role like that, so presumably they wanted him to be wooden. Similarly it's part of the plot that Kellerman and Bond didn't have the strongest relationship with Reilly. (Normal boys don't steal and eat unknown substances from their father's laboratory.) It's twice as hard to understand what's going on when you're having to ask yourself whether something was a clue or just the episode being rubbish.
Weird stuff will happen. This goes without saying. However it's also oddly random, like the car crash of three different horror stories. There are animal mutilations. There are machines going out of control. There are signs that this might be becoming a ghost story, e.g. possession. There are maggots. (Eh?) I couldn't tell if I was watching Poltergeist
, The Omen or The Midwich Cuckoos
. Even now, I'm not sure if the driver of this horror story was supposed to be Blackstad's powers, Reilly from beyond the grave, the parents' dissociation with their children or even the dangers of genetic engineering. (Yes, I realise that this is 1980 and the scriptwriter's probably never even heard of it, but even so on one of its levels this episode's still fumbling at the same buttons. It's like pre-scaremongering.)
To answer my own question, I think the answer to "who was responsible" is "all of them". Trying to insist on one answer is not only reductive, but leaves plot holes. Blakstad is clearly involved on some level, even if we don't know exactly how, but what changed the inscription on the gravestone right at the end?
All this might sound like a mess, but in fact the episode works. I quite liked it. I certainly couldn't predict where it was going. You could identify antecedents for individual elements of its story, but I believe the way those ingredients have been combined here is unique. I also think the performances work, for a given definition of "work". The only actor I'd call bad is Reilly and he's not troubling us for long. Bond and Kellerman are completely convincing on their own terms and if you find yourself wondering why they're not as emotionally connected to the situation as they might be, that's deliberate. Finally, as for Blakstad, you can't simply call him unrealistic, because there's never been a boy like him. He's either autistic, plugged into another world or just missing parts of his brain.
Mind you, his stress is wrong in "after they died, mother and father were cremated."
In addition I wouldn't put all the blame on Bond and Kellerman for Reilly. Undoubtedly they could have reached out to him more, but it's also possible that he was a self-obsessed teenager who thought the world revolved around him and lacked a balanced overview. His parents' real mistake might have lain in not correcting that.
The Doctor Who round-up's low this time, with only Tariq Yunus (The Robots of Death) and a gravedigger played by Geoffrey Beevers (The Ambassadors of Death, The Keeper of Traken). In other matters, they don't break the Hammer House of Horror Nudity Rule.
Would I recommend it? I'm not sure. It's intriguing, but my online browsing suggests that it's also not particularly popular. Personally though I found plenty to think about and the odd interesting quirk, such as Blackstad's toy rabbit or the African and Indian perspectives of Bond's international observers. I'm far from convinced the plot holds together, but it's sufficiently rich and layered that it could take you a while to decide. At any rate, it's original.