On a bright Saturday in early spring, Detective Gemma Monroe responds to a missing person call at Lost Lake, near the small town of Cedar Valley, Colorado. With its sapphire waters and abundance of wildflowers, the lake is a popular camping destination in the summer. But for now, ice still grips the lake and snow buries the flowers.

When Gemma arrives at the shore, she meets three friends who have been camping there: the fourth of their group, Sari Chesney, has disappeared in the night without a trace. Sari is an assistant curator at the local museum, which is set to host a gala that night - a project she's worked on for months and would never intentionally miss. As Gemma begins to understand the complex dynamics of the supposedly close-knit friendship group, she realises that more than one person is lying to her - and that the beautiful, still waters of Lost Lake may hide more terrible secrets.