Journey to Babel

While transporting dignitaries to an important peace conference on the planet Babel, the Enterprise is pursued by a mysterious vessel and, after a quarrel between the Vulcan ambassador, Sarek (Spock's father), and the Tellarite ambassador, the latter is killed, with Sarek as the main suspect. Kirk is then attacked and badly injured by the assassin, exonerating Sarek but requiring that Spock take charge of the Enterprise. An ethical dilemma arises when Sarek falls ill and needs Spock's blood for a risky life-saving operation. To his mother's distress Spock puts his duty ahead of his father's life. Kirk and Bones find a way to allow Spock to assist in the operation, while a still-injured Kirk tries to uncover the assassin's plan and engages the attacking starship.

Summary from Wikipedia, CC BY-SA 4.0.