Gunnery Sergeant Roberta Draper, called Bobbie, stands guard with a Martian Marine squad outside a Ganymede greenhouse dome. Across the ice, UN Marines guard a nearly identical dome, and Bobbie’s squad passes the dull patrol time by nicknaming and watching them. The narration establishes that after Eros, Earth and Mars have turned Ganymede, the food source of the Jovian system, into a tense divided front watched by warships overhead.

As Ganymede passes into Jupiter’s shadow, Bobbie leads Privates Hillman and Gourab and Corporal Travis around the dome. Bobbie’s aging powered armor develops a hydraulic problem in the left knee, and Hillman stops to repair it. During the delay, their outpost calls to say shots have been fired at the UN position and orders them back for lockdown.

Bobbie’s squad races toward the outpost as a general alert sends Bobbie’s armor into combat mode. At the Martian firing line, radio and satellite links suddenly fail under jamming. Lieutenant Givens tells Bobbie that the Martians cannot contact the oncoming UN Marines and have authorization to fire if the UN troops come within half a kilometer.

Bobbie studies the seven incoming figures and realizes the six UN Marines in front are not attacking but fleeing from a seventh figure behind them. Magnification reveals a chitin-covered, bare, inhuman creature with grotesque hands and a huge distorted head. Bobbie opens fire and tries to warn the line, but the creature tears through the UN soldiers, shrugs off bullets and cannon fire, and kills Hillman indirectly by hurling an armored UN Marine into him.

The creature reaches the Martian line, killing Marines and tearing open the Yojimbo combat mech to throw Sa’id from the cockpit. Bobbie and the survivors retreat toward the dome, but Gourab is caught at the door and killed. The creature turns on Bobbie, revealing bright electric-blue eyes, then suddenly mutates further and explodes, throwing Bobbie across the ice. As Bobbie fades toward unconsciousness, warships begin firing overhead, and Bobbie is unable to report that the UN Marines had been fleeing something else.