Spock and McCoy are at odds--irreconcilably. The exact cause of their argument is not stated outright. McCoy turns out to have a parasite which has been causing his unusual behavior (in the end, he even says that he doesn't care if Spock dies, which tips M'Benga off that something is wrong). Due to their argument, Spock is left without emotional support, which triggers a--fatal--condition the Vulcans call 'mental debility'. Spock does recover, though--the cure turns out to be expressing emotion, which McCoy manages to induce him to do.