Months after surviving the inferno, ATF Forensics Chemist Dr. Cassian Holt and K9 Handler Jonah Briggs are a team in the field?and in secret. Their new bond is tested when a case in rural Missouri unearths a sophisticated disposal pipeline, using the state's sprawling limestone caves to make victims vanish.

Racing a flash flood that will obliterate the crime scene, Cassian descends into the oppressive darkness and makes a horrifying discovery: this isn't a tomb. It's a pantry. The killer is still holding victims alive. Now trapped underground by a rockslide and rising floodwaters, they must turn a watery grave into an escape route. Can they trust each other to find a way out, or will the weight of the mountain bury their future forever?