Welcome to HMP North Fern – the newest maximum security women’s prison in England. No one can go anywhere without the system knowing about it – all doors are coded to the electronic cuffs that everybody wears. You cannot go anywhere you shouldn’t go and your every movement is recorded.

Cara Lockhart, nicknamed The Butcher, is the latest inmate to arrive at North Fern, but she manages to find some sort of routine in life behind bars, even electronic ones. She even makes friends with her cell mate – life is almost bearable.

Until one night when the cameras go out for a couple of seconds – and when they come on, Cara’s cell mate has been shot in the head. The door never opened, no one else entered the cell – Cara is obviously the guilty party, despite never moving on the tape before and after the camera loss. And the fact that the gun is nowhere to be found…

“The new novel from the king of the locked room mystery” – well, that’s quite a claim. Not the new novel bit, the bit about McGeorge being the king of the locked room mystery.

As you know, I’m old school on this and the current use of the phrase “locked room mystery” does rather get on my goat. And Then There Were None, and the countless modern isolated group mysteries are not locked room mysteries. They are closed circle mysteries, which doesn’t really catch on, but they are not locked room mysteries. For decades, a locked room mystery involves a crime committed inside a location that was apparently inaccessible to the culprit. There are other impossible crimes that can fit under the same banner, but it’s the impossible bit of the definition that’s crucial.

Chris McGeorge seems to have got his aforementioned reputation from Guess Who, his first novel, an enjoyable thriller/mystery involving five people waking up locked in a room with a body with nobody knowing which of them killed the victim. It’s an entertaining book, but it’s not a locked room mystery in the accepted sense of the phrase.

The set-up for Inside Out is, however. Fans of classic mysteries will see echoes of Carter Dickson’s masterpiece The Judas Window in the set-up, where the suspect awakes to find himself locked inside a room (from the outside) with the victim. The Judas Window has a cunning solution to what happened – completely unbelievable if you think about it too hard, but that’s the Golden Age for you.

But be warned – this book is not The Judas Window.

It’s far more of a thriller than a mystery novel, and for the most part, it’s a very entertaining one. Cara’s predicament is gripping, and McGeorge cleverly drip-feeds parts of the back story to lead the reader to the big picture of what is going on, both with Cara and with North Fern in general. It’s one of the most effective uses of the possibly-unreliable narrator that I’ve seen for a good while, and you really develop a concern for the character, despite, initially, being unclear of what earned her the nickname Butcher – probably not her impression of Karl Urban – and how sane she really is. The pacing is excellent, the villains suitably nasty, although it is somewhat undermined by the obviousness as to who is who and the supervillain-level bonkersness of the overall scheme. I think both of these are deliberate choices, but as I say, this is a thriller more than a classic mystery.

And the locked room… there is really only one way to do a locked room when it is under camera surveillance, and it’s never satisfying. As such, it’s a shame that this seems to be a major selling point in the blurb.

So, if you want a thriller (especially if you don’t mind it going a bit OTT at the end) then this is recommended. But if you want a classic locked room mystery, well, someone’s going to reprint The Judas Window at some point, surely.

Inside Out is out in paperback and ebook on Thursday 29^th October from Orion. Many thanks to NetGalley and the publisher for the review copy.