After losing contact with the planet Deneva (population 1 million) for more than a year, the USS Enterprise goes to investigate. Uhura's attempts to make contact at all frequencies, including private GSK783 subspace frequency 3, but without success. Spock consults the computer and discovers that waves of mass insanity have swept along a straight path through planetary systems Beta Portalin, Lavinius 5 (200 years ago), Theta Cygni 12, and Ingrahem B (2 years ago). Deneva (colonized a century ago and one of the most beautiful planets in the area) is the next planet on this path, and Kirk expects the worst for his brother Sam and his family.