Stardate 5725.3: The Enterprise is delivering newly designed equipment to Memory Alpha, the repository of all knowledge in the Federation. The officer in charge is Lt. Mira Romaine, who Scotty is infatuated with. As they approach the planet, they detect a storm of an unidentified sort. It approaches the ship and penetrates the shields, and everyone seems paralyzed with shock. Mira is particularly affected, seemingly somehow linked to the storm. It then withdraws and everyone snaps out of their paralysis, except Mira who collapses to the floor, stunned. A strange garbled noise emerges from her throat despite the fact she appears to be unconscious. She wakes up and says that she's fine but McCoy takes her to Sickbay. Spock determines that the disturbance was a storm, while Uhura desribes how she was paralyzed. The other crew suffered other forms of sensory deprivation, including speech loss, and they suspect it may have been an attack.
Scotty goes to Sickbay with Mira, and thinks her reaction was just a matter of space nerves. Mira refuses to cooperate with McCoy's examination and gets increasingly angry. Meanwhile, the crew determines the storm is heading directly for Memory Alpha, which has no defenses as a peaceful planet. The Enterprise arrives as the storm departs, and they sense no lifesign readings. Kirk takes down Spock, McCoy, and Scotty while Mira stays in Sickbay, where she sees a vision of a dead technician.
The landing party arrives and finds that the entire staff is dead and the computer core is irrevocably damaged. They find one survivor, although she is close to death. She makes the same garbled noises as Mira did on the bridge, then her face glows with multi-colored light. Kirk has Mira beam down while McCoy determines the technician died because her entire brain patterns were wiped. The other staff were destroyed when different centers of their brains were destroyed. Kirk tells Mira what happened and she storms by and sees the dead technician from her earlier vision. She tells them they have to get back to the ship, as the storm is returning. Spock tries to reassure her but the ship contacts them and informs them the storm is coming back. They beam up but something interferes with the transport, trapping Mira in transit. Scotty manages to rematerialize her.
They leave orbit but the storm pursues them and Spock determines that the storm is no natural phenomena. While Scotty tries to reassure Mira, the Enterprise is unable to evade or outrun the storm and Spock determines that it is a conglomeration of at least ten living beings. Kirk tries to contact it and warn it off, but it accelerates and moves in front of them.
Mira starts to feel the storm's presence in Engineering and she has another vision. On the bridge, Kirk orders a phaser barrage which affects the creature… and in Engineering, Mira collapses. Kirk orders another blast but Scotty calls to let them know the phaser blasts are affecting Mira, and another shot will kill her. Kirk calls off the attack and Spock suggests they find an environment deadly to the creature that doesn't affect Mira.
Kirk calls a conference with Mira and McCoy displays a brain scan he ran of her after the alien attack. Spock notes that it is a duplicate of the sensor scan he took of the aliens. He concludes that the aliens are merging their minds with Mira.
The alien colony creature calls in while Scotty explains the visions Mira had earlier. She admits she's had one other vision: of Scotty dying. As Scotty tries to reassure her, the entities catch up to the Enterprise. Kirk tells Mira to let the aliens function through her so they can control the moment and defeat them.
They take Mira to the pressure chamber in Sickbay while the creatures penetrate the shields and merge with Mira before she can enter. She glows with energy and emits the garbling noises, but tries to fight back. She collapses and the entities speak through her. They say that they have searched for millennia for someone they could live their lives through, and they came from Zetar. Their planet was destroyed and the entities are the consciousness of the last hundred Zetars. A sudden disaster struck them down but their life force continued The Zetars did not wish to kill but their resistance destroyed them. Kirk vows not to let them survive at the cost of another. Mira tries to fight back without success.
They try to move her into the pressure chamber and Scotty carries her in, although the Zetars strike at him just as he succeeds. Scotty survives the attack and they neutralize the gravity, then increase the atmospheric pressure at a rate of one atmosphere per second. The Zetars finally emerge from her and then are destroyed by the pressure. They gradually lower the pressure while Scotty stands watch.
Later, Spock, McCoy, and Kirk meet to discuss the case and they think that Mira will emerge better from the experience, and that she should return to duty. Kirk calls Scotty who agrees that Mira is ready for duty. They set course for Memory Alpha as Kirk notes that it's a historic force: all three of them in agreement.