On its way to the Memory Alpha planetoid, storehouse of all cultural history and scientific knowledge of the Federation, sensors detect a strange storm. The storm is traveling at a speed of Warp 2.6, indicating that it cannot be a natural phenomenon. The storm heads right for the Enterprise, penetrating the shield and attacking different centers of the brain in different crew members (Uhura could not move her hands, Chekov could not turn his eyes, and Sulu could not speak). Lt. Mira Romaine, aboard to oversee transmission of data newly gathered by the Enterprise to Memory Alpha, seems the hardest hit. Much to the concern of Scotty, who is head over heels in love with her, Mira faints. When McCoy revives her, she makes strange growling noises and then seems to be normal again. However, she then refuses to cooperate with McCoy when he attempts to examine her.
The storm then heads for shieldless Memory Alpha, killing all those aboard and burning out the central memory core. Kirk, Spock, McCoy, and Scott beam down to see what damage has been done, and see a scene of which Mira has had a premonition (a dead technician). The landing party finds one technician alive, but she begins making strange growling noises, turns a series of colors, and then dies of brain hemorrhage. Mira is then beamed down, and she warns everyone to return to the Enterprise because the storm is returning. Scans from the Enterprise confirm this, and the landing party returns to the ship. Mira is suspended in transit, but is subsequently beamed aboard. Further scans of the ``storm'' indicate that it is composed of 10 life units. Kirk tries to communicate with it using the universal translator, but receives no response. As the storm draws closer, Mira has another vision. Kirk fires phasers across the storm's path, but again elicits no response. He the fires at the storm, but this action causes Mira to crumple, causing Scott to become alarmed and forcing Kirk to desist from further firing.
A thorough analysis of Mira's record shows her to responses to be extremely flexible response to new learning situations. A Steinman analysis shows nothing unusual, but a hyperencephalogram shows Mira's brain circuitry pattern to have been altered to match those from the last tracking of the aliens. This seems also to explain her visions, and Mira reveals that she had one additional vision: that of Scotty dying.
In an attempt to rid Mira of the alien influence before the aliens attack again, Kirk rushes her to a gravity/pressure chamber. The aliens attack too soon, however, and Mira becomes completely possessed. Speaking through Mira, the aliens identify themselves as the last survivors of the planet Zetar. They have had to discard their bodies and have been searching for a millennium for one such as Mira's in which they can live out their lives. Before Mira's consciousness can be completely subjugated, Scotty puts her in the pressure chamber. Here, the aliens are killed and Mira is freed.