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The Undetectables
The Unsolvable Case
The queer witchy fantasy murder mystery series concludes in one last mystery for Mallory, Cornelia and Diana, as they have to solve their biggest, and most important case of all - who killed Theodore. Perfect for fans of supernatural mysteries and cosy crime by authors such as Ben Aaranovitch, Josiah Bancroft and Tammie Painter.
It's three days until Samhain, and the Undetectables are going to die. No longer taking magical cases (not that anyone would hire them), they've well and truly given up the ghost.
Meanwhile, Theodore (the relinquished ghost in question) is on the run. Wanted in connection with the Undetectables' last case, he's hiding in the last place anyone would think to look, keeping secrets (badly) and forming unlikely alliances (slightly less badly).
When a copycat killer emerges and history appears to be repeating itself, the Undetectables are called up for one last case. Mallory, Diana and Cornelia find themselves in a final race against the clock – but this time, the whole of Occulture is at stake.
It's three days until Samhain, and the Undetectables are going to die. No longer taking magical cases (not that anyone would hire them), they've well and truly given up the ghost.
Meanwhile, Theodore (the relinquished ghost in question) is on the run. Wanted in connection with the Undetectables' last case, he's hiding in the last place anyone would think to look, keeping secrets (badly) and forming unlikely alliances (slightly less badly).
When a copycat killer emerges and history appears to be repeating itself, the Undetectables are called up for one last case. Mallory, Diana and Cornelia find themselves in a final race against the clock – but this time, the whole of Occulture is at stake.
495 pages, Kindle Edition
Published September 22, 2026
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Displaying 1 - 3 of 3 reviews
October 2, 2026
⭐️⭐️⭐️⭐️½ — The Unsolvable Case by Courtney Smyth
Thank you to Netgalley and the publisher for trusting me to read this ARC in exchange for my honest review!
There is something incredibly difficult about ending a series readers have spent multiple books becoming attached to. You have to solve the mystery, honor everything that came before it, give the characters room to grow, and somehow make saying goodbye feel satisfying. Courtney Smyth manages to do all of that while delivering what feels like the darkest and most emotionally significant case the Undetectables have faced.
The Unsolvable Case brings us back to Mallory, Cornelia, Diana, and Theodore at a point where things are very different from where this group began. The Undetectables aren’t exactly thriving, Theodore is on the run, and with Samhain only days away, there is already this underlying sense that something terrible is coming. Then a copycat killer appears, old patterns begin resurfacing, and suddenly the group is dragged into one final mystery with consequences that could affect all of Occulture.
What I enjoyed most about this book is that the mystery actually feels worthy of a series finale. It isn’t simply another case dropped into the same familiar formula. There is history behind it. There are consequences. There are connections to everything these characters have survived previously, which gives the investigation considerably more weight.
And Smyth understands something I always appreciate in a mystery: the reveal matters, but the people experiencing the reveal matter just as much.
The friendship between Mallory, Diana, and Cornelia has always been the heart of this series, and that becomes even more apparent here. Their dynamic still contains the humor, chaos, and banter that make these books fun, but underneath it is this very real sense of people who have grown together, hurt together, and continued choosing one another anyway. By the final book, they no longer feel like characters assembled to solve mysteries. They feel like a genuine found family.
Theodore also adds another layer to the story because his situation creates a second source of tension running alongside the main investigation. He isn’t simply hanging around as the group’s resident ghost anymore. He has secrets, problems of his own, and a much larger role in what is happening. The parallel storylines eventually begin tightening around one another, which gives the latter portion of the book that wonderful “okay, now everything is starting to connect” feeling.
I also loved the atmosphere. This entire series lives in that strange sweet spot between cozy mystery, supernatural fantasy, murder investigation, and slightly unhinged magical chaos, and The Unsolvable Case leans heavily into all of it. Setting the story around Samhain adds an appropriately eerie edge without turning the book into outright horror.
The worldbuilding remains one of Smyth’s strongest elements as well. Magic isn’t simply decorative here. Magical forensics, ghosts, supernatural politics, and the larger world of Occulture are woven directly into how the mystery operates. It makes the investigation feel different from a traditional detective novel while still giving readers the clues, suspects, theories, and misdirection we want from a mystery.
My one hesitation is that there are moments where the story feels a little dense. There is a lot happening because Smyth is simultaneously solving a present-day mystery, dealing with consequences from earlier books, developing several character arcs, expanding the supernatural stakes, and closing an entire series. Readers coming primarily for a fast murder mystery may occasionally feel the pace slow beneath all of that.
But for readers already invested in these characters, I think that extra space largely works because this isn’t simply about discovering who committed a crime.
It is about finishing their story.
And when the reveals begin coming, Smyth does a good job of making previous details feel important rather than throwing in twists simply for shock value. That’s the kind of mystery writing I appreciate most—the moment when you realize the answers were being built in front of you even if you weren’t quite putting them together yet.
Most importantly, the ending feels emotionally earned.
There is tension. There are surprises. There are moments where you genuinely wonder how these characters are going to get themselves out of the situation they’ve landed in. But beneath all of that is a story about friendship, identity, grief, loyalty, and what happens when the people who helped you survive one version of yourself are standing beside you as you become someone new.
If you’ve followed The Undetectables from the beginning, this feels less like simply reading another mystery and more like getting one last night with people you’ve grown attached to.
Funny, eerie, queer, emotional, mysterious, and just chaotic enough to remain unmistakably The Undetectables.
I’m sad to leave Wrackton behind.
But if this really had to be the final case?
This was one hell of a case to go out on.
Did it fool me? Enough that I absolutely had moments where Smyth had me looking in one direction while the story was quietly preparing something somewhere else.
And you already know that’s exactly what I want from my mysteries. 🔎🕷️
Thank you to Netgalley and the publisher for trusting me to read this ARC in exchange for my honest review!
There is something incredibly difficult about ending a series readers have spent multiple books becoming attached to. You have to solve the mystery, honor everything that came before it, give the characters room to grow, and somehow make saying goodbye feel satisfying. Courtney Smyth manages to do all of that while delivering what feels like the darkest and most emotionally significant case the Undetectables have faced.
The Unsolvable Case brings us back to Mallory, Cornelia, Diana, and Theodore at a point where things are very different from where this group began. The Undetectables aren’t exactly thriving, Theodore is on the run, and with Samhain only days away, there is already this underlying sense that something terrible is coming. Then a copycat killer appears, old patterns begin resurfacing, and suddenly the group is dragged into one final mystery with consequences that could affect all of Occulture.
What I enjoyed most about this book is that the mystery actually feels worthy of a series finale. It isn’t simply another case dropped into the same familiar formula. There is history behind it. There are consequences. There are connections to everything these characters have survived previously, which gives the investigation considerably more weight.
And Smyth understands something I always appreciate in a mystery: the reveal matters, but the people experiencing the reveal matter just as much.
The friendship between Mallory, Diana, and Cornelia has always been the heart of this series, and that becomes even more apparent here. Their dynamic still contains the humor, chaos, and banter that make these books fun, but underneath it is this very real sense of people who have grown together, hurt together, and continued choosing one another anyway. By the final book, they no longer feel like characters assembled to solve mysteries. They feel like a genuine found family.
Theodore also adds another layer to the story because his situation creates a second source of tension running alongside the main investigation. He isn’t simply hanging around as the group’s resident ghost anymore. He has secrets, problems of his own, and a much larger role in what is happening. The parallel storylines eventually begin tightening around one another, which gives the latter portion of the book that wonderful “okay, now everything is starting to connect” feeling.
I also loved the atmosphere. This entire series lives in that strange sweet spot between cozy mystery, supernatural fantasy, murder investigation, and slightly unhinged magical chaos, and The Unsolvable Case leans heavily into all of it. Setting the story around Samhain adds an appropriately eerie edge without turning the book into outright horror.
The worldbuilding remains one of Smyth’s strongest elements as well. Magic isn’t simply decorative here. Magical forensics, ghosts, supernatural politics, and the larger world of Occulture are woven directly into how the mystery operates. It makes the investigation feel different from a traditional detective novel while still giving readers the clues, suspects, theories, and misdirection we want from a mystery.
My one hesitation is that there are moments where the story feels a little dense. There is a lot happening because Smyth is simultaneously solving a present-day mystery, dealing with consequences from earlier books, developing several character arcs, expanding the supernatural stakes, and closing an entire series. Readers coming primarily for a fast murder mystery may occasionally feel the pace slow beneath all of that.
But for readers already invested in these characters, I think that extra space largely works because this isn’t simply about discovering who committed a crime.
It is about finishing their story.
And when the reveals begin coming, Smyth does a good job of making previous details feel important rather than throwing in twists simply for shock value. That’s the kind of mystery writing I appreciate most—the moment when you realize the answers were being built in front of you even if you weren’t quite putting them together yet.
Most importantly, the ending feels emotionally earned.
There is tension. There are surprises. There are moments where you genuinely wonder how these characters are going to get themselves out of the situation they’ve landed in. But beneath all of that is a story about friendship, identity, grief, loyalty, and what happens when the people who helped you survive one version of yourself are standing beside you as you become someone new.
If you’ve followed The Undetectables from the beginning, this feels less like simply reading another mystery and more like getting one last night with people you’ve grown attached to.
Funny, eerie, queer, emotional, mysterious, and just chaotic enough to remain unmistakably The Undetectables.
I’m sad to leave Wrackton behind.
But if this really had to be the final case?
This was one hell of a case to go out on.
Did it fool me? Enough that I absolutely had moments where Smyth had me looking in one direction while the story was quietly preparing something somewhere else.
And you already know that’s exactly what I want from my mysteries. 🔎🕷️
October 1, 2026
Be gay, solve crime, take naps
If you have read the three books in the story The Undetectables, The Undead Complex and the The Unfathomable Curse I cannot imagine why you wouldn't want to find the end of this tale The Unsolvable Case. Everything I loved from the last three books is here . The incredible world-building further revealed and developed with especial attention on the founding myth of the Occulture, the focus here is on our favourite Samhain ghost our cat ear wearing Theodore who after the stunning reveal in the last book we thankfully get to read his side of that story. Throw in a countdown which sees The Unfathomable Curse due to end our plucky detectives Mallory, Diana and Cornelia its quite the pacy read with a satisfying resolution to all the mysteries. I appreciated after the resolution the author gave us a couple of visits to where these characters were and what they were doing.
I look forward to reading more of Courtney Smyth.
If you have read the three books in the story The Undetectables, The Undead Complex and the The Unfathomable Curse I cannot imagine why you wouldn't want to find the end of this tale The Unsolvable Case. Everything I loved from the last three books is here . The incredible world-building further revealed and developed with especial attention on the founding myth of the Occulture, the focus here is on our favourite Samhain ghost our cat ear wearing Theodore who after the stunning reveal in the last book we thankfully get to read his side of that story. Throw in a countdown which sees The Unfathomable Curse due to end our plucky detectives Mallory, Diana and Cornelia its quite the pacy read with a satisfying resolution to all the mysteries. I appreciated after the resolution the author gave us a couple of visits to where these characters were and what they were doing.
I look forward to reading more of Courtney Smyth.
Review of advance copy received from Publisher
I am so sad that this series is over because I love these characters so much. They've become a part of my life and now what am I meant to do without them?!
This was the perfect send off for these characters and I honestly loved every minute of it. It was dark, tense and mysterious, having me hooked from the beginning and audibly gasping when things were revealed. I love when a book surprises me and Smyth does such an amazing job of it.
If you've read my reviews for the previous books, you will know that one of my favourite things about this series is the characters and representation throughout. I think a lot of people will be able to see themselves in these characters as they are so relatable and realistic.
The Undetectables Series is one of my all time favourite series and I absolutely recommend picking it up, especially if you love mysteries in a paranormal world! I am so sad to see this series finish but am so happy to have read it. I cannot wait for whatever Courtney Smyth writes next! She has easily become an auto-buy author for me!
This was the perfect send off for these characters and I honestly loved every minute of it. It was dark, tense and mysterious, having me hooked from the beginning and audibly gasping when things were revealed. I love when a book surprises me and Smyth does such an amazing job of it.
If you've read my reviews for the previous books, you will know that one of my favourite things about this series is the characters and representation throughout. I think a lot of people will be able to see themselves in these characters as they are so relatable and realistic.
The Undetectables Series is one of my all time favourite series and I absolutely recommend picking it up, especially if you love mysteries in a paranormal world! I am so sad to see this series finish but am so happy to have read it. I cannot wait for whatever Courtney Smyth writes next! She has easily become an auto-buy author for me!
Displaying 1 - 3 of 3 reviews