The Federation starship Enterprise reaches the planet Gideon for diplomatic discussions. The Gideons allow only Captain Kirk to visit their planet. Surprisingly, after beaming down, Kirk finds himself on an empty Enterprise.

First Officer Spock learns Kirk has yet to arrive on Gideon, but Gideon Ambassador Hodin refuses a search team. Neither Starfleet Command nor the Federation bureaucracy offers assistance.

Now alone on the deserted ship, Kirk discovers a mysterious bruise on his arm and meets Odona, a beautiful young woman. She shares that her home planet is incredibly overpopulated, and being alone is a rare luxury. Kirk and Odona share a passionate kiss but remain oblivious to the faces appearing on a nearby viewscreen.

Soon, Kirk hears strange noises outside the ship and glimpses a crowd of onlookers. Confronting Odona, she denies any knowledge of the situation. She suddenly faints, and Kirk takes her to the sick bay.

Ambassador Hodin reveals that Odona, his daughter, has been infected with Vegan choriomeningitis from Kirk's blood. Hodin plans to infect the Gideon population with the virus to reduce their lifespans and ease overpopulation. Kirk is to provide the virus, while Odona's death will inspire future volunteers.

Realizing Hodin's deception, Spock defies Starfleet's orders and returns to Kirk's original coordinates, finding a fake Enterprise. He locates Kirk and Odona, and the trio returns to the actual Enterprise[3]. Dr. McCoy treats Odona, who decides to return to Gideon with the virus. Kirk forgives her deception.