The story follows Martha Kivelsen, an astronaut who is in a moon rover crash while exploring the surface of Jupiter’s moon Io. The rover is destroyed, so Kivelsen decides to make a sledge from the rover's parts and drag her partner Burton’s body back to the lander before her air supply runs out. She has no time to sleep so she takes drugs to keep her awake and give her energy, but they cause her to hallucinate. Along the way she hears a voice in her radio claiming to be Io, which is actually a machine, using Burton's body to communicate. When Kivelsen is trapped on the wrong side of a lake, Io claims it can help her cross it, and sulfur crystals make a narrow bridge over the lake. However, where Kivelsen is nearly at the lander, it is destroyed by a quake, stranding her without air. Io states it may be able to give her and Burton a form of immortality, and Kivelsen directs Io to use its flux tube to tell the rest of the Solar System about her discovery. Then she accepts the offer and jumps into a pool of molten sulfur. Whether or not she is hallucinating remains ambiguous.
The narration also describes Io's scenery by referring to The Wizard of Oz. While Kivelsen thinks about why she is retrieving Burton's body, she mentions the Baltimore Catechism.
