Metamorphosis

Kirk, Spock and McCoy are flying Federation Commissioner Nancy Hedford back to the Enterprise by shuttlecraft. She has caught a rare disease and needs urgent treatment, but a strange energy cloud diverts the shuttle to a planetoid. There they meet a man named Cochrane, who turns out to be Zefram Cochrane, the discoverer of the space warp, believed dead for 150 years. A being he calls the Companion found him dying, restored his youth and has kept him alive ever since. It brought the shuttle down so that he would have company. Kirk and Spock discover that the Companion is female and in love with Cochrane, who is repelled by the idea. With Hedford close to death, the Companion merges with her, curing her and allowing it to love Cochrane as a human woman. Because the joined being cannot leave the planetoid, Cochrane chooses to stay with her, and the others leave without him.