Lieutenant Capp is integrating Royal Navy tech onto a battered ship from the Old Earth fleet when she gets orders to cross the so-called Roach Frontier and investigate a potentially hostile alien race. The insectoids have survived a genocidal war, and are expanding aggressively toward human territory.
She arrives to find a navy warship already lurking in the system, with Captain McGowan of HMS Peerless preparing a preemptive strike against an insectoid mining colony. Capp sees hints that the aliens hope to avoid conflict, but McGowan ignores her misgivings and attaches her to a force of marines tasked with snuffing the aliens before they can spread.
The insectoids may not have initiated the fight, but they mount a vicious response to the mechanized troops landing in their midst. The deeper Capp penetrates their territory, the more she realizes this is a fight the marines may not be able to win, as she discovers a secret brewing deep within the insectoid colony.