The Enterprise discovers a planet that defies the conventional theories of planetology, so Kirk, Sulu, McCoy and D'Amato (a geologist) beam down to investigate. As they depart, a mysterious woman appears in the transporter room and attacks the chief there. Then the Enterprise is hurled nearly a thousand light years away! The strange female apparition reappears, and demonstrates that she is a grave threat to both stranded crew and the ship...

Rigelian Fever rages about the Enterprise; on a small planet named Holberg 917G, the crew has discovered a source of the only cure, ryetalin. Kirk, Spock and McCoy beam down, and discover the planet is the private property of Flint. At first uncooperative, Flint relents and permits the crew to gather the material. But events soon have them wondering about Flint's real goals, and his mysterious ward, the enchanting Rayna.