Everybody who watches Abducted in Plain Sight, the latest true crime phenomenon on Netflix, has the exact same question. Minus the inevitable handful of swear words: "What were the parents thinking?

Try putting yourself in their shoes for a minute. Your 12-year-old daughter has gone horse riding for the day with a trusted family friend, it's now night time and they still haven't come home. (It's also 1974, so you can't just call or text.)

Do you ring the police straight away? Do you assume they must have got a flat tyre or something and wait until the morning? Or do you do what these guys did and sit on your hands for a full five days before finally reporting it to someone?

I'll do just about anything to avoid talking on the phone and, like the mum, I also live in perpetual fear of ever inconveniencing anybody – "I didn't want to get all these people worked up over nothing," she remembers thinking on the third or fourth day. But I reckon even I would have called the cops a bit sooner.

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This is just the start of it, the first five minutes of a documentary that is constantly escalating into darker, weirder, more upsetting and unbelievable territory. It's a story told mostly by the victims, which on one hand provides a certain level of comfort (they're still alive and basically okay now) but on the other hand this is also what makes it so harrowing.

The Brobergs were a typical church-going family in small-town Idaho when they first met Robert "B" Berchtold and family in 1972. He was affable, charming, and the two families became close. He was also, it's so glaringly obvious to see in hindsight, completely obsessed with the Broberg's eldest daughter Jan and sexually abused her for years.

"I don't know how we could have been so gullible when there were so many red flags," the dad wonders about it now. In a devastating bit of editing, we cut straight to the detective who investigated Jan's first kidnapping in 1974: "I found the Brobergs to be naive." That's the understatement of the century.

Like most true crime documentaries there might come a point where you start to question the morals and ethics of reliving a family's decades-old trauma for entertainment. Whether that's after the first kidnapping, the second kidnapping, the faked alien abduction, the marriage in Mexico – everybody has their own limits. With this one at least the victims have a voice, some agency. Their strength and forgiveness offers some kind of hope at the end of it all.

After years and years of murders, with a detour through weird cults last year, Abducted in Plain Sight shows there's still plenty of grim places for the true crime genre to go. It would be nice to think of this as a one-off, that "child abduction" isn't about to become the next big trend, but don't be so sure – a Netflix series about the disappearance of Madeleine McCann has been rumoured for years.