Spock, the first officer of the USS "Enterprise", begins to exhibit unusual behavior and requests that he be granted leave on his home planet Vulcan. Captain Kirk and Chief Medical Officer Dr. McCoy, having witnessed one of their friend's outbursts, agree and Kirk diverts the ship to Vulcan. En route, Kirk receives orders from Starfleet to travel to Altair VI to represent the Federation at the inauguration ceremony for the planet's new president. Though Kirk instructs the crew to set course to Altair VI, Spock secretly changes course back to Vulcan. Kirk confronts Spock, who claims to have no memory of ordering the course change.