Writer: Stephen W. Carabatsos
Dir.: Herschel Daugherty

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Christine Chapel: Majel Barret

Kartan: Dave Armstrong
Peter Kirk: Craig Hundley
Aurelan Kirk: Joan Swift
Yeoman Zahra Jamal: Mauriska

Stardate: 3287.2

Deneva. The planet is home to captain Kirk's brother, George Samuel Kirk, and family. The Enterprise heads for Deneva because an interplanetary mass insanity spreads like an epidemic and distress calls reach them from all over the planet. As they come closer, they overhear the transmission of a ship, which flies into the sun. The pilot rather destroys himself than bearing an enormous pain. He experiences the end of the pain and total relief just before he realizes that he is killed by the sun. 

They beam down to investigate and find deserted streets. At Kirk's brother's home they find Sam dead. Aurelan, his wife, is dying and their son, Peter, unconscious. Flat, round and plate-sized aliens who intervene with the human nervous system cling themselves onto their host and manipulate the person to do what they want – all by putting the victim under tremendous pain. While examining a creature Spock is attacked by another which infiltrates his nervous system. He suffers unbearable pain and before he manages to control himself in spite of the pain, he experiences a moment where the creature has that much influence on him that he tries to take over the Enterprise and land her on the planet. 

McCoy tries anything to fight them, but to no avail. All the time Spock suffers immense agony. Against captian's orders he tries to be beam down to capture one of the creatures for further study. Scott stops him, but Spock manages to talk Kirk into letting him go. The creatures seem to resist any kind of treatment until Kirk, remembering the starship flying into the sun, speculates that they might resent intensive light. They plan to try to expose all the infected people to higly intense light and Spock volunteers to test the procedure on himself. 

McCoy would like to wait for further results first, "Jim, he's the best first officer in the Fleet!" he warns Kirk, but time is of an essence since the creatures rapidly kill the people on the planet. The impact of immense light works, but blinds Spock. By now the test show that ultraviolet light would have been enough. Spock staggers towards a chair, bumping into a desk – he seems to be blind. The planet now is thoroughly covered with ultraviolet light by satelites and the planet freed from the aliens. To everybody’s surprise Spock comes onto the bridge with full eyesight. The blindness has been temporarily because an extra eyelid the Vulcans have to protect them from the intensive light of their sun had saved his eyesight. Now, McCoy is preoccupied, "Don't tell him I said he's the best first officer in the Fleet ..." - he whispers in Kirk's ear, but Spock's Vulcan hearing doesn't forgive.