After disobeying orders to save the planet of Alsea, Captain Ekatya Serrado has returned home to something less than a hero's welcome. At her side is Dr. Lhyn Rivers, now the foremost authority on a culture that fascinates and terrifies.

Ekatya and Lhyn share a secret: they are tyrees, linked by an Alsean empathic bond that should be biologically impossible for Gaians. Exposure could destroy Ekatya's career. But when both women are drawn into a high stakes political game, their tyree bond may be all that stands between them and a terrible fate.

In Catalyst, the fourth book of the Chronicles of Alsea, the bonds of love, friendship, and family are redefined. The intersection of the Alsean and Gaian cultures has profoundly changed both—and become a catalyst for miracles.