After a rockslide exposes a long-sealed research bunker on Azure Mountain in northern New Mexico, ranger Mara Ellison discovers a bloodstained field journal and a brass compass etched with initials. Investigative podcaster Cal Redding arrives chasing the cold case of Dr. Owen Park, a hydrologist who vanished during the 1999 drought scandal. When a cipher in the journal points to a shaft beneath the Blue Needle chairlift, a masked snowcat blocks the only service road, forcing them onto a treacherous ridge trail.
Cut off by an incoming storm and a fast-moving wildfire, Mara and Cal follow coordinates scratched on a 1978 topo map to a limestone tube locals call the Heart. Inside, an illegal conduit drains the aquifer toward a shuttered copper mill, and someone with a silenced rifle will do anything to keep it running. Armed with an avalanche beacon, a stuck ham radio, and two expired flares, they gamble on the forgotten funicular tunnel beneath Switchback Gulch, where every footfall echoes like a countdown.