An excerpt from a fictional guidebook, Backcountry River Running: A Primer, notes that the Garnet River is called the “River at the End of the World” because there is no road access for over one hundred miles after the launch point at Boundary Creek.

Seven years later, on July 11, 2025, Kaia is on a ten-passenger van heading to the Garnet River for a five-day women’s retreat with Idaho Rapids Unlimited. She is grieving the death of her husband, Parker, who died at age 28 after becoming wedged in a narrow cave passage and could not be extricated during a failed rescue. She had originally planned the Garnet River trip with Parker as a joint celebration of their 30th birthdays. Kaia feigns sleep to avoid group icebreakers led by the cheerful guides, Macy and Gracie. At the launch point, Macy and Gracie introduce themselves to the whole group, explaining that they aren’t related despite their rhyming names and that both attend the University of Idaho, where they study recreation therapy, using outdoor activities to help people heal from trauma. The group meets the two male guides: Josh, nicknamed “River Jesus,” and a new, clumsy guide named Bryan and nicknamed “Flip-Flop,” a name he dislikes. During introductions, a guest named Allie, a high school librarian wearing a “Crime Junkies” t-shirt, asks how close they are to Camas Creek. Bryan replies that they will camp there the next night. Allie expresses excitement, referencing rumors about Kellen Stroop—alluding to the murders that occurred at Camas Creek seven years earlier. The guides, particularly Josh, become tense at the mention of Stroop’s name and abruptly end the conversation, directing everyone to prepare the rafts.

In a safety briefing, Josh demonstrates how to float in the “Idaho Easy Chair” position, and Gracie demonstrates the technique for pulling someone back into the raft. The group then loads into the rafts. Kaia is in a raft with Macy and several other women, including Deborah and her sister, Beth. Kaia finds herself opening up slightly to Deborah, who is divorced and on the trip as a gift from her daughters. As they row, Kaia reflects on how much Parker would have loved the majestic canyon. Allie again brings up the Camas Creek murders, referencing a movie about the event, Blood in the Water, prompting Macy to quickly redirect the group’s focus to rowing. Despite the morbid talk, Kaia feels a sense of purpose on the river, resolving to be present for both herself and Parker’s memory.

Kaia begins to enjoy the rhythm of rafting and the beauty of the canyon. The group spots mountain goats on the cliffs and successfully navigates a small rapid. They watch as the new guide, Bryan, struggles to pilot his gear boat through the same section. Macy explains to the women that Bryan earned the nickname “Flip-Flop” because he showed up to guide training in flip-flops—inappropriate and unsafe footwear for river rafting. At Fight Night, a separate hazing event where guides push each other off stacked rafts, Josh was forced to wear his flip-flops, and he lost every match. As a further humiliation, he was then assigned the duty of emptying the camp’s portable toilet, nicknamed the “groover,” for the rest of the summer. As dusk falls, they make camp on a small beach just before a dangerous Class-Four rapid known as the Devil’s Stairs. Macy explains that Camas Creek, the next campable beach, is 30 miles away. After the guests go for a swim, Kaia wanders down a short trail to view the Devil’s Stairs. Watching the violent, churning water, she reflects on Parker’s death and realizes the worst has already happened to her, leaving her with nothing left to fear.

After dinner, the women gather around the campfire. Kaia sits near Imani, who shares photos of her young, twin boys. As some of the women talk, Kaia overhears Allie discussing the Camas Creek murders and the movie Blood in the Water with another guest, Nikki. A third woman, Pam, asks for the full story. Macy and Gracie, clearly uncomfortable with the topic, leave the fireside. Josh, however, agrees to tell the story.

Josh recounts the events of the Camas Creek massacre from seven years ago, explaining that one of the guides who died was a friend of his older brother. A guide for Idaho Thrills, Kellen Stroop, murdered 15 people on a trip. The sole survivor was a 17-year-old boy named Leo Marsden, who played dead under his mother’s body in a raft. When Stroop came to throw Leo’s mother overboard, Leo struck him with an oar; Stroop went overboard but caught a D-ring, and Leo then strangled him with a rescue-rope bag while pushing his head underwater. Leo navigated the rest of the river alone to find help. The group discusses the conspiracy theory, popularized by the film Blood in the Water, that Stroop might have survived. Allie, who moderates a Reddit forum on the case, confirms that police exhumed Stroop’s body and verified his death. Kaia asks how one man managed to kill 15 people, and Josh ominously replies that this is the scariest part of the story.

An excerpt from a book titled The River Guide’s Almanac advises that all river guides should carry a knife. The text specifies that the blade should be three to four inches long and is intended for use in emergencies to cut through tangled lines, rope, or webbing.

Josh publicly confronts the new guide, Bryan, forcing him to show everyone his knife. To the group’s shock, the knife—with a camouflage-patterned hilt and three inlaid pearl crosses—is identical to the one used years ago by the mass murderer Kellen Stroop. A defensive Bryan claims he bought it recently and was unaware of its significance. Allie then recounts how Stroop first murdered a female guide, causing panicked guests to flee onto the river in rafts in the pitch dark, while Nikki separately details the bodies found downriver. Stroop killed several more people, including Leo’s mother. The other rafters who escaped were later found dead miles downriver. While some drowned, their bodies had gruesome and inexplicable injuries, including missing hands and heads, that a knife alone could not have caused. The group is deeply unsettled by the story, and Bryan promises to get rid of his knife.

Kaia has difficulty sleeping and thinks of Parker, remembering a TED Talk about “Third Man Syndrome, ”in which people in life-threatening situations sometimes hallucinate the presence of a friend or loved one who guides them to safety. She has a vivid dream in which Parker is guiding her raft through a treacherous rapid. He is about to say something important, but the only sound that emerges from his open mouth is a shriek, which yanks Kaia from her sleep. She realizes the screaming is coming from Allie’s tent. The other campers and guides gather outside Allie’s tent, unsure what to do. From inside the tent comes another scream.

Allie explains that she had a night terror, which she attributes to the scary stories told earlier. Josh, one of the guides, comes to check on the commotion and sends everyone back to bed. Before returning to her own tent, Kaia checks on Allie, who confides that her night terrors actually began after her second round of in-vitro fertilization failed. Kaia returns to her tent and goes to sleep. An hour later, she is woken again, this time by the faint sound of footsteps and the noise of a strap being unbuckled near the rafts. Assuming it’s one of the guides, she eventually falls back asleep.

An excerpt from The River Guide’s Almanac states a key principle for survival: When in the backcountry, one must operate under the assumption that outside help will not arrive. This fact is presented as the important foundation upon which every decision should be based.

Kaia wakes to a normal morning at the riverside camp. As breakfast is served, Macy angrily interrupts, demanding to know the location of something from a gear raft. In a hushed argument with fellow guides Josh, Gracie, and Bryan, she reveals that the satellite phone is missing from the emergency box. Bryan and Gracie are certain it was packed, but Josh insists he saw it during his security check before everyone went to bed. He concludes someone must have stolen it while they slept.

Overhearing their conversation, Kaia tells the guides that she heard someone moving around by the rafts in the middle of the night, about an hour after Allie had a nightmare. The guides speculate that a scared guest might have taken the phone for reassurance. Concluding they cannot leave without it, they decide to postpone breaking camp until the satellite phone is found.

Macy informs the guests that the satellite phone is missing. Allie and Nikki are visibly horrified, explaining that the phone also mysteriously disappeared during the Camas Creek massacre. Josh angrily cuts them off. The guides lead a two-hour search of the camp—dry bags are dumped out, gear boats torn apart, and tents turned inside out—but find nothing. To avoid causing panic, Josh lies to the group, claiming they must have accidentally left the phone at the launch point. Macy gives Kaia a sharp look, silently warning her not to contradict his story. Kaia stays quiet, realizing that someone on the trip likely stole the phone, leaving them completely cut off from the outside world.