Spock is becoming increasing irrational, even threatening to break McCoy's neck! He can't or won't explain this behavior to Kirk, only asking to return to Vulcan. It's a request Kirk cannot grant, given the Enterprise's other business. Until McCoy tells him if Spock continues to deteriorate, he will die in days! Kirk makes the decision to take his friend back to his home world, where he learns that Spock's problem has to do with marriage...