A shuttlecraft was transporting Assistant Federation Commissioner Nancy Hedford to the USS Enterprise to be treated for Sukaro's disease, a life-threatening condition before she could continue her efforts for peacemaking. Then, unexpectedly, an energetic field emerged in the path of the shuttlecraft, dragging it to a nearby celestial body with a climate similar to that of Earth. As a result, all communications were rendered impossible, and the shuttlecraft could not be operated.

Not long after, someone claiming to be Cochrane appeared. He told the group that he had been stranded on the planet for some time, and a dampening field was blocking their systems from operating. Cochrane brought them to a safe house constructed from parts of his ruined vessel. As they visited, Captain Kirk and Spock, alongside Dr. McCoy, saw an illuminated form similar to the phenomenon that brought them to the planet. Cochrane named this entity "The Companion" and explained that as an elderly man, he had taken a final voyage, expecting to pass away in space. Still, the entity saved his malfunctioning ship and made him young again while preserving him ever since. The Starfleet members were astounded to discover he was Zefram Cochrane, the inventor of warp drive. Cochrane then exposed why they were taken to the planetoid: he had revealed to the Companion that he would die without the presence of his species, thinking it would let him go. Instead, the Companion abducted the shuttlecraft.

When the being assaults Spock as he maintains the shuttle, Spock concludes that the creature is made up of a great deal of electrical power. Kirk and Spock endeavor to deactivate the Companion with a makeshift electric disruptor, yet the Companion reacts ferociously, and only Cochrane's help prevents Kirk and Spock from being slain.

Spock modifies the shuttle's universal translator to make conversation with the energy force possible, and it is revealed that the force is a female and has a fondness for Cochrane. The Companion expresses that it has halted the aging process of everyone on board and will keep them there forever to keep Cochrane company. Cochrane, however, is revolted by the thought of an intimate relationship with a being from another species and refuses the Companion's love. Finally, Hedford, on the brink of death, expresses her wish to love and be loved before she passes away.

Cochrane again calls upon the Companion, and Kirk explains that they are too dissimilar to be in a relationship. The Companion ponders what it would be like to be a human and then vanishes. Soon after, Hedford appears outside the shelter, fully recovered, and they understand that the Companion has merged with Hedford's body, which was about to expire. This way, Hedford, and the Companion can have a chance at love. Cochrane is overjoyed as he talks about his ambitions to explore the universe, but the Companion/Hedford reveals that its energy is bound to the planetoid and can't leave for a long time. Thus, Cochrane decides to stay with her out of love and appreciation, claiming they will have "many years" together.

Cochrane implored Kirk and his crew not to reveal his presence on the asteroid for him and the Companion/Hedford to have privacy. When McCoy asked who would carry out Nancy Hedford's mission, Kirk shrugged and replied, "The Federation will be able to find another woman to stop the war."