Four days alone in the desert. Except they're not as alone as they think.

A year ago, Katie and her cousin Aster survived a night that left their world and easy friendship fractured. Desperate to heal and leave the past behind them, they tackle four days of hiking in the Utah backcountry. But the desert they've loved for years has tricks up its sleeve.

An illness, an injury, and a freak storm leave them short on confidence and supplies. When they come across a young couple with extra supplies on the trail, they're grateful and relieved—at first. Riley exudes friendliness, but everything about her boyfriend Finn spells trouble. That night, after some chilling admissions about Finn from Riley, Katie and Aster wake to hear the couple fighting. Helpless and trapped in the darkness, they witness Riley's desperate race into the night, with Finn chasing after.

In the morning, they find the couple's camp, but Riley and Finn? Vanished. Katie is sure Riley is in trouble. And with help a two-day hike away, they know they are the only ones who can save her before something terrible happens. The clock is ticking and their supplies are dwindling, but Katie and Aster know they have to find Riley before Finn—or the desert—gets to her first.