After disobeying orders and saving the planet of Alsea from invasion, Captain Ekatya Serrado returns home a hero and renegade, alongside Dr. Lhyn Rivers, now the foremost authority on a culture that fascinates and terrifies. They share a secret: they are tyrees, linked by an Alsean empathic bond that should be biologically impossible for two Gaians. The secret could cost Ekatya her career, but when both women are drawn into a deadly political game, their tyree bond may be all that stands between them and the dangerous enemies they have made.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.