There are few things more unsettling than a place that refuses to give up its secrets.

In Riley Sager’s The Unknown, that place is New Avalon, a remote island in Vermont where five women disappeared in 1926. Their dresses were found hanging from an old oak tree, but the women themselves were never found.

A century later, actress Marin Keane arrives on the island to film a movie about the disappearance.

The timing is already uncomfortable.

Then the cast begins disappearing too.

Published August 4, 2026, The Unknown combines Sager’s familiar fascination with secrets, isolated settings, and shifting explanations with a stronger supernatural presence. It is a mystery about missing women, a thriller about a film crew trapped on an island, and, at times, something much closer to horror. Penguin Random House — The Unknown by Riley Sager

The result is atmospheric, strange, and often genuinely difficult to put down.

It is also not without problems.

My verdict: The Unknown is an effective, eerie thriller with an excellent premise, but its atmosphere is ultimately stronger than some of its characters and its final revelations.

The Unknown by Riley Sager: Quick Review

Title: The Unknown
Author: Riley Sager
Published: August 4, 2026
Publisher: Dutton
Pages: 368
Genre: Mystery, Thriller, Horror
Pages & Prose Rating: ⭐ 4/5
Reviewer: Delia Marchetti

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Quick Take

The Unknown works best when it lets the island do the frightening.

The isolation, the history of the missing women, the old diary, the séances, and the knowledge that the film crew is deliberately recreating a tragedy create a steady feeling of dread.

Sager is less successful when the story has to slow down and make its characters feel as compelling as the mystery surrounding them.

Still, the central question is strong enough to carry the novel:

What really happened on New Avalon—and why does it seem to be happening again?

What Is The Unknown About?

In 1926, five women disappeared from New Avalon, an isolated Vermont island that had been home to a community of spiritualists.

There were no bodies.

There were no clear answers.

There were only five dresses hanging from an old oak tree.

One of the missing women, Daisy Rue, left behind a diary that helped turn the disappearance into a legend. Her account suggests that a séance may have gone terribly wrong and that something unknown may have been brought into the world.

A hundred years later, actress Marin Keane is cast in a movie about the disappearance.

The production takes the unusual step of bringing the cast to New Avalon itself.

They aren’t simply filming there.

They’re attempting to recreate the original events.

The actors study the missing women. They wear period clothing. They participate in séances. They immerse themselves in the history of the island.

Then the line between research and reality begins to disappear.

Someone—or something—seems to be repeating the events of 1926.

And Marin has to figure out what happened before the island claims another victim.

The Setting Creates the Dread

For me, the most effective part of The Unknown is New Avalon.

Sager understands that isolation can be more frightening than almost any monster.

The island doesn’t need to constantly produce shocks. It only needs to remind you that the characters are cut off from everything familiar.

The water surrounding them becomes a barrier.

The old buildings become evidence.

The history of the missing women hangs over every room.

And because the cast has deliberately come to the island to recreate the original tragedy, every strange occurrence feels more ominous than it otherwise might.

The characters are essentially asking to become part of the story they’re investigating.

That is an excellent premise for suspense.

The Mystery Has the Right Ingredients

Sager gives us several mysteries at once.

What happened to the five women?

What does Daisy Rue’s diary actually tell us?

Was the séance responsible for the disappearances?

Is there a supernatural presence on New Avalon?

Or is someone manipulating everyone from the shadows?

The novel knows how to keep these questions alive.

Just when one explanation begins to feel convincing, another possibility appears.

That is classic Riley Sager territory, but The Unknown adds enough supernatural uncertainty to keep the formula from feeling entirely familiar.

The best moments are the ones where I wasn’t sure which explanation I wanted to be true.

That uncertainty is where the novel has its teeth.

The Film Within the Story Is More Important Than It First Appears

The movie-production angle isn’t just an excuse to get a group of characters onto the island.

It gives the novel one of its most interesting ideas.

The original disappearances were a tragedy.

A hundred years later, they’re entertainment.

The missing women have become characters.

Their suffering has become a story.

Their island has become a filming location.

And Marin and the other actors are being asked to recreate events that may have destroyed real lives.

That creates an uncomfortable question:

How much of someone else’s tragedy can we turn into entertainment before we lose sight of the people who actually suffered?

This is one of the reasons The Unknown has more going on beneath its supernatural mystery than the premise initially suggests.

The Dual Timeline Works

The story moves between the historical mystery and the modern-day film production.

Daisy Rue’s diary becomes an important bridge between them.

The further Marin investigates, the more the two timelines begin to overlap.

That structure creates one of the book’s strongest tensions.

We’re not simply asking what happened in 1926.

We’re watching people in the present recreate the conditions that led to it.

The closer they get to understanding the original disappearance, the more dangerous their own situation becomes.

It’s a smart way to make the past feel active rather than simply historical.

Where the Book Is Strongest

The atmosphere

This is the obvious winner.

New Avalon feels isolated, old, and wrong in exactly the way a supernatural thriller needs.

The premise

A movie crew recreating a century-old disappearance on the exact island where it happened is difficult to resist.

The uncertainty

Sager keeps several explanations alive long enough for the reader to question their assumptions.

The slow-building dread

The novel doesn’t need to rely on constant shocks.

Much of its tension comes from waiting for something to go wrong.

And because we already know that five women disappeared on this island, we know exactly what kind of wrong we’re waiting for.

Where The Unknown Falls Short

The characters don’t always have the same presence as the mystery.

Marin is given enough personal history to make her involvement meaningful, but the surrounding cast can sometimes feel more functional than fully developed.

That matters because the emotional impact of a disappearance depends partly on how much we care about the person disappearing.

There are moments when I found myself more invested in what New Avalon was hiding than in who was discovering it.

That’s not fatal for a thriller.

But it prevents The Unknown from reaching the emotional depth its premise occasionally promises.

The Final Act Is the Biggest Risk

This is where readers are likely to divide.

Sager builds a complicated web of possibilities, and the ending has to untangle it.

Some readers will enjoy the revelations precisely because they force a second look at what came before.

Others may feel that the final explanations push the story too far.

That distinction is important.

A twist isn’t automatically satisfying simply because it surprises you.

The real test is whether, after the surprise fades, you can look back and think:

Of course. The clues were there.

The Unknown gets close to that feeling in places, but not every revelation lands with the same force.

The final stretch is ambitious, and I suspect readers will be arguing about it for a while.

That isn’t necessarily a weakness.

But it is something to know before you begin.

Is The Unknown Actually Scary?

It can be—but its strongest weapon is unease rather than outright terror.

This is a supernatural thriller with horror elements rather than a relentless horror novel.

The séances, disappearances, strange events, and isolation create an increasingly uncomfortable atmosphere.

If you like books where you can feel something approaching before you can see it, The Unknown has plenty to offer.

If you want graphic horror or nonstop scares, you may find it comparatively restrained.

For me, the most effective moments are the quiet ones.

The moments when the characters notice something they cannot explain.

The moments when the island suddenly feels too quiet.

The moments when a historical detail starts looking less like history and more like a warning.

What Is The Unknown Really About?

The supernatural mystery is the engine, but the novel is also interested in storytelling itself.

The five missing women became a mystery.

The mystery became a legend.

The legend became a book and eventually a movie.

And now people are physically recreating it.

That progression raises questions about memory and exploitation.

Who gets to tell a tragedy?

Who gets to profit from it?

And what happens when the people recreating someone else’s story begin experiencing the same nightmare?

The book doesn’t abandon its thriller ambitions to explore those questions, but they give the mystery an additional layer.

How Does The Unknown Compare With Riley Sager’s Other Books?

Fans of Riley Sager will recognize several familiar ingredients:

• isolated locations
• secrets from the past
• suspicious characters
• unreliable information
• escalating danger
• major revelations
• a final act designed to change how you understand the story

But The Unknown leans particularly hard into the supernatural.

That makes it a good choice for readers who enjoy the atmospheric side of Sager’s work.

It may be less appealing to readers who prefer his stories when the mystery stays firmly grounded in reality.

The New York Times has described the novel as something of a “victory lap” for Sager, particularly rewarding readers familiar with his earlier work.

For longtime fans, that familiarity can be part of the fun.

For newcomers, it may make more sense to approach The Unknown simply as an isolated supernatural mystery.

Who Should Read The Unknown?

I’d recommend it if you enjoy:

• Atmospheric mystery
• Supernatural thrillers
• Isolated settings
• Missing-person stories
• Dual timelines
• Séances and spiritualism
• Slow-building dread
• Uncertain explanations
• Riley Sager’s twist-heavy storytelling

You’ll probably enjoy it most if the mystery matters as much to you as the characters.

Who Might Not Enjoy It?

I’d be more cautious if you prefer:

• Deeply character-driven fiction
• Completely realistic mysteries
• Extremely fast pacing
• Straightforward plots
• Horror centered on graphic violence
• Perfectly logical explanations for every mystery

The book asks you to accept some ambiguity and to enjoy the journey toward the answer.

Final Verdict

The Unknown has a premise that does exactly what a good thriller premise should do:

It makes you want to know what happens next.

Five women disappeared from New Avalon.

A hundred years later, actors arrive to recreate their story.

Then the disappearances begin again.

That’s enough to carry a reader a long way.

What makes the novel memorable is the atmosphere surrounding that premise. New Avalon feels increasingly oppressive, the historical mystery remains intriguing, and Sager understands how to keep uncertainty alive.

Where the book falls short is in the emotional weight of its characters and, for some readers, the final explanations.

But I don’t think those weaknesses erase what works.

If you’re looking for a late-summer thriller with a creepy island, a century-old mystery, supernatural possibilities, and enough twists to keep you questioning your assumptions, The Unknown is a strong choice.

Pages & Prose Rating: 4/5 ⭐
Our rating reflects the book’s overall experience, with particular attention to suspense, pacing, clues, atmosphere, and whether the twist earns its impact. Learn more about [how we rate books at Pages & Prose].
Reviewed by Delia Marchetti, Mystery & Thriller Contributor at Pages & Prose.

The bottom line: The Unknown isn’t Riley Sager at his most emotionally complex, but it may be one of his most atmospheric mysteries—and New Avalon is a place I suspect readers will remember long after they’ve closed the book.

Frequently Asked Questions

The Unknown: A Novel

Published August 4, 2026, The Unknown combines Sager's familiar fascination with secrets, isolated settings, and shifting explanations with a stronger supernatural presence. It is a mystery about missing women, a thriller about a film crew trapped on an island, and, at times, something much closer to horror. Penguin Random House — The Unknown by Riley Sager

URL: https://pagesandprose.com/the-unknown-by-riley-sager-review/

Author: Riley Sager

Editor's Rating:
4.5