However, fans of the book will notice there have been quite a few changes made to that novel's plot and characters. For instance, in the original story, Karen only comes in right at the end, while here she is front and centre right from the start.
The series has been adapted by Emer Kenny, who also stars in the series as Karen's friend River Wilde. Kenny spoke exclusively with RadioTimes.com, and explained why she had to make such major changes when bringing the story to the screen.
She said: "This show is really different to the book, because there's a lot of stuff that just wasn't gonna work on screen. The first half [of the book] is set in the 1970s, the second half is set in 2004 – so I was gonna have to update it to 2022 and therefore the past moved forward to the 1990s.
"And that changed so much of the story, because policing was different between the '70s and the '90s, and also, the characters change. If you are 28 or 29 in 2004, you're a different generation to 28-29 now.
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"But also in the book, Karen is only in, I think, about 20 pages of a 600-page book, she comes in at the end and she's quite instrumental, but she's not the protagonist in any way. But then Val takes her through the rest of the books on a bigger journey."
Kenny has previously said that it was suggested she move on to the second book for the first season of the show, to more easily include Karen in the narrative. However, here she explained: "I love this book. I think I read it about eight times over the process.
"I made tons of notes, I worked out what it was exactly that I wanted to keep, what was really essential about the story, which was the two timelines, the central character of Karen, the group of university students – I loved that, because I love writing friendships, I love writing relationships – and then a few of the regulars.
"Those were the core things that I wanted to stay true to from Val. But then I kind of just threw everything up and went, 'Right, now I've got to write a TV show' because most of the people coming to a TV show haven't read the book. So you have to make sure it fits in the TV landscape as its own unique thing."
Another major change made for the series was the inclusion of new character Bel Richmond, who creates a podcast about the unsolved case Karen ends up investigating, the 1996 murder of Rosie Duff. The podcast brings the case back into discussion and is the reason it gets reopened by the police.
Kenny explained that this was included because she "was really worried about it being another 'dead girl show', because we see a lot of murdered women on screen".
She continued: "I wanted a way to comment on that and become quite meta with it, in that I could have the podcast commenting on the case, I could have Karen commenting on the podcast, and it's just ways for me to layer up the discourse around what we're showing and what we're doing and the issues that are at play.
"True crime podcasts are such a phenomenon. It felt like it'd be almost strange to do a show about an old case surrounding a girl who doesn't make it home without in some way having podcasts come in, because they are so present in that world in our time, so it felt like a good fit to be able to have that as a device."
The series also sees a major change to the group of boys we follow from the 1990s, who all become the prime suspects in Rosie's murder. In the book there are four of them, but in the series there are just three.
Explaining this change, Kenny said: "Two of the characters felt quite similar and had a lot of overlap and it felt natural to merge them. Also, one of the characters moves to America, and he doesn't feel as present in the book, so it just made sense to boil them down to three characters in the end."
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