The Federation starship Enterprise is on its way to Memory Alpha, a planetoid housing the Federation's central library. A storm-like phenomenon, moving at warp speed, is also heading toward the planetoid. The Enterprise intercepts the storm, infiltrating the ship and impacting the crew members' nervous systems. Lieutenant Mira Romaine, assigned to Memory Alpha, collapses due to the storm's effects.

The storm continues to Memory Alpha, with the Enterprise following, devasting the station's computer core. Finally, Captain Kirk, Spock, Dr. McCoy, and Mr. Scott arrive at the station to assess the damage. Meanwhile, Romaine experiences visions of dead bodies at Memory Alpha.

The team discovers severely damaged archives and deceased staff, except for one survivor who exhibits similar symptoms as Romaine and dies from a brain hemorrhage. Kirk brings Romaine to the station, where she is horrified to see the scene from her vision, and warns of the storm's return.

Scans reveal a storm is a group of life forms. Kirk attempts communication but has yet to receive a response. After unsuccessful warning shots, Kirk orders a full attack, causing Romaine to experience pain. Scott pleads with Kirk to stop.

In a briefing, McCoy reports Romaine's altered brain wave pattern, which matches the storm's sensor data. The team concludes that the aliens are trying to control Romaine's body. They devise a plan to let the aliens partially control Romaine before exposing her to high atmospheric pressure.

Before placing her in the pressure chamber, the aliens take control of Romaine, identifying themselves as survivors from planet Zetar. They plan to inhabit Romaine's body for the remainder of their existence. Scott manages to get Romaine into the pressure chamber, eventually driving out and seemingly destroying the aliens.

Following the crisis, Spock, McCoy, and Scott agree that Romaine is fit for duty and assign her to oversee the restoration and repair of Memory Alpha's archives.