Enterprise penetrates the Delphic Expanse hunting the Xindi who attacked Earth, threatening to exterminate humanity. Captain Archer and Trip attempt to learn about the mysterious and antagonistic race. But their zeal to capture a Xindi leads them into a trap.

Enterprise's ongoing pursuit of the Xindi leads to an investigation of an abandoned Xindi vessel on a jungle world. A highly contagious mutagenic virus infects Archer, Hoshi, and Reed and rapidly mutates them into the planet's primal and extinct native beings, the Loque'eque, but T'Pol is unaffected. As Phlox searches for a cure, Trip and T'Pol negotiate with aliens determined to kill all those infected.