AllMovie

This episode of the popular science fiction television series finds the Enterprise battling a contagious form of insanity that has already lead to the devastation of several planets. The starship finds the illness has spread further than initially feared when they discover that nearly all of the population of the planet Deneva -- including Captain Kirk's brother -- has wiped itself out. Further exploration reveals that the insanity is caused by an extraterrestrial parasite, which next infects Mr. Spock. Dr. McCoy must race against time and Spock's growing madness to analyze the creature and find a cure, even if it means sacrificing Spock's life in the process.