The exploration vessel Spindrift is sent to frontier space around the planet TUBOY to investigate the disappearance of unmanned scout unit NO. 00511. The ship is attacked on arrival and barely escapes into the shadow of a nearby satellite, retrieving the scout's black box before breaking off.
Analysis of the black box reveals two alarming facts: the alien civilization on TUBOY has been programmed to destroy all life, and having intercepted data from NO. 00511, those robots now know of Earth's existence and are already constructing a massive space battleship in the planet's deep ocean. The crew resolves to destroy the battleship from the inside before it can launch. Three pilots in cutting-edge all-terrain space units called EXECTOR are sent to the surface—two are destroyed, leaving the last pilot to complete the mission alone.