In an attempt to establish diplomatic relations with planet Gideon, Kirk beams down to the sensor-shielded planet using coordinates supplied by Gideon ambassador Hodin (875-020-079). Gideon has refused to let any ambassadors beam down or to permit any sensor scans, but has indicated to the Federation that their planet is virtually a paradise. However, they agree to let Kirk beam beam alone.
When Kirk materializes, he finds himself still on the transporter pad aboard the Enterprise. To his great surprise, he appears to be completely alone and there is no sign of the crew. After noticing a bruise on his arm, Kirk encounters a girl named Odona. Odona denies any knowledge of how she got there or what is going on. She tells Kirk her last memory is of being in an overcrowded auditorium struggling for air, and how wonderful it is to have such freedom of movement.
Back aboard the real Enterprise, Spock is attempting to determine why ambassador Hodin denies the fact that Kirk has ever been beamed down. His attempts to obtain authorization to search for Kirk are foiled when Starfleet bureaucrats insist that he contact the Federation diplomats in the Department of Planetary Treaties, and vice versa. After beaming up and down Hodin's assistant Krodak from the council chamber (coordinates 875-020-709) to test the transporter, Hodin refuses to allow Spock to beam down, despite the fact that he had previously agreed to do so.
Meanwhile, on the deserted Enterprise, Odona tells Kirk that there is no place on her planet which is not filled with people, and that the inhabitants would kill or die for the privilege of being alone. While Kirk is busy kissing Odona, he does not notice that the ship's viewing screen has become filled with the faces of people. After leaving the bridge, Kirk hears a strange sound outside the ship and opens the viewing port. To his surprise, he briefly sees the faces of a crowd, which quickly fade into stars.
Kirk begin to suspect that something is very wrong, but when he confronts Odona, she complains that she feels very strange, then collapses. Kirk carries her towards sickbay, but is met by Hodin and taken into custody. Hodin tells Kirk that Odona is his daughter, and that she has been infected with Vegan coreomenengitis from Kirk's blood. Hodin explains that the germ-free atmosphere and an increasingly long life span on Gideon have created a vast overpopulation problem, and that he hopes to bring it under control by introducing disease.
When Kirk asks why the population was not sterilized, Hodin says the the organs of the people of Hodin regenerate themselves. When Kirk then asks Hodin why the people of Gideon have not practiced birth control, Hodin replies that the people of Gideon believe life is sacred and that the love of life is the greatest gift, and that they therefore cannot interfere with the creation of life. Hodin tells Kirk that he must stay to provide the virus to infect the people of Gideon and bring the population under control. Kirk points out that Odona need not die, and that there is furthermore no need for him to stay since if Odona survives, her blood can provide the virus. However, Hodin tells Kirk that Odona's voluntary death at such a young age will bring forward dedicated young volunteers to also be infected.
By this time, Spock has discovered that the two sets of coordinates with which he has been provided do not match. After Admiral Fitzgerald continues to refuse to let Spock beam down, Spock disobeys orders and beams down to the first set of coordinates. He discovers the exact replica of the Enterprise to which Kirk was beamed and locates Kirk. Spock overpowers the guards, and he and Kirk beam up with Odona to the real Enterprise. As he is leaving, Spock quips to the ambassador, ``Your excellency, please do not interfere; I already have one serious problem to resolve with upper echelons.'' McCoy heals Odona and she is beamed back to Gideon where her blood can now serve as the source of Vegan coreomenengitis for her people.