In a war of extermination with the vast Melkonian Empire, Earth sends ships to establish a remote star colony that can serve as humanity's insurance policy. The fleet squadron accompanying them dies in a clash with superior enemy forces; only the intervention of two powerful intelligent Bolo combat machines participating in the expedition saves most of the ships. The remaining hidden Melkonian ship blows up one of the machines and lands troops after the people on the planet they have chosen. This landing can be destroyed only at the cost of heavy losses and the decisive contribution of the only remaining Bolo.