After a grueling journey, Wren Foster and her team finally reach the vicinity of Project Solstice, hidden deep within the perpetually storm-lashed "Shroud." But the legendary facility is no abandoned relic. In The Frozen Heart of Solstice, they find it protected by automated defenses and strange, cold-resistant guardians—sentinels from a bygone era.

Their path is further complicated by another, more desperate faction of survivors, led by the charismatic but dangerous Commander Joric Steel. This ruthless group is also searching for Solstice, believing it holds a weapon, not a cure, and they will stop at nothing to claim it. Inside the labyrinthine complex, Wren and Dr. Gideon Shaw race against time, uncovering the full, horrifying truth of the ice age's origins: a catastrophic miscalculation or a deliberate act of geoengineering gone terribly wrong.

The book culminates in a tense, multi-front confrontation where Wren's team must not only activate the "thawing" technology but also prevent it from falling into the wrong hands or causing further, irreversible damage. The fate of the world rests on their shoulders, in the frozen heart of humanity's greatest mistake.