Artemis's Mother manifests as a sentient planetary entity formed entirely of gelatinous substance, an immense and unidentified life form. This being functions as the collective home for Artemis and her numerous sisters, constituting a cohesive biological family within its structure. The entity drifts unintentionally onto the Galaxy Express 999's trajectory; a collision embeds the space train within its surface. Railway authorities classify it solely as a navigational obstacle, planning to dislodge the train using a destructive Vibration Wave, unaware of its sentience.

Tetsuro Hoshino intervenes following his encounter with the dying Artemis, who returns to the entity after traumatic experiences in a machine body. Through this interaction, he recognizes the planet is not merely an obstacle but a living mother. This revelation pivots the narrative towards protecting the entity from the planned countermeasure. Its existence and the resulting crisis serve as a narrative device exploring maternal themes, contrasting its unconditional, passive nurturing with a mechanistic society devaluing organic life.

The story concludes as Artemis perishes; her remains dissolve into reunion with the mother entity's gelatinous form. This act resolves the immediate threat, prompting the entity to move voluntarily away from the railway tracks, freeing the Galaxy Express 999 to continue its journey. The entity's departure underscores its fundamental role as a silent embodiment of maternal bonds and sanctuary, posing a thematic challenge regarding empathy for life forms beyond human comprehension.