Nancy Hedford
Eleanor Donahue

Zefram Cochrane
Glenn Corbett

The shuttlecraft Galileo containing Kirk, Spock and McCoy is returning to the Enterprise after collecting Commissioner Nancy Hedford who has contracted the lethal Sakuro's Disease and urgently needs treatment. Before it can rendezvous with the Enterprise Spock's sensors detect an 'energy cloud' heading towards them at warp speed. It soon envelops the shuttle and drags it off to a remote planetoid. The crew is powerless to stop it or to make any sort of communication.

On the surface, the shuttle will not operate although Spock's tricorder can read nothing wrong with its systems. It does, however, show that the 'cloud' is somewhere on the planetoid's surface. A man comes to greet them and explains that nothing will work on the surface due to an energy-damping field effect. He identifies himself as Zefram Cochrane, tells them that he has been marooned there for a very long time and invites them to his home, constructed from the remains of his spaceship.

The sparkling energy cloud appears outside the dwelling and Cochrane reveals that it is an entity called 'The Companion', telling them how, 150 years ago, he had flown his craft into space to die and was found by the entity which saved and rejuvenated him. It had brought him here and looked after him ever since. Cochrane guesses that they were brought to this place to stop him dying of loneliness. It slowly dawns on Kirk that their host is actually none other than the man who invented warp drive and was the first human to make contact with the Vulcan race.

Meanwhile, Nancy Hedford's condition is deteriorating and Kirk asks Cochrane if he will request the Companion's help. The cloud surrounds Cochrane but, when it leaves, he says there is nothing the Companion can do to help her. The cloud travels to the shuttle, which Spock is trying to re-activate, and attacks it, burning out most of its circuitry and knocking Spock out with a heavy charge of electricity. He is discovered by McCoy and helped back to Cochrane's house where he sets about constructing a machine to neutralise the cloud's energy. Cochrane is very concerned that they may kill it but they have no choice.

When the machine is used on the Companion it is found to be mainly ineffective and it attacks Kirk and Spock who are only saved by Cochrane calling it off. They eventually manage to communicate with it, by using a modified universal translator from the shuttlecraft, and discover that it is both female in gender and besotted with 'the man' Cochrane. He is horrified by this revelation, and runs away from the group.

Nancy Hedford watches all this and is saddened by the thought that she, too, has never known a man's love. Kirk tells the companion that Cochrane cannot return its love as they are too different and that he will die if he cannot be freed. The Companion enters the body and mind of the dying Commissioner, restoring her health and giving them both an opportunity to experience what they have always wanted.

Cochrane is able to accept and return the love of the Companion in its new form, but it cannot now leave the planet or it will die. She and Cochrane agree to let the shuttlecraft leave if they can be left alone together on the planetoid to grow old and die in peace. The Enterprise has finally located the missing craft and takes it back on board when it leaves the planetoid. Kirk keeps his promise, and Nancy Hedford is logged as having died en route and being buried on the planetoid.