A conference of ambassadors becomes a murder mystery, and Spock must reckon with his father.
The Enterprise carries Federation delegates to a conference on Babel to decide whether the dilithium-rich Coridan system may join. Among them is Vulcan ambassador Sarek, Spock's estranged father, and his human wife Amanda. After a public quarrel with Sarek over Coridan, Tellarite ambassador Gav is found dead from a Vulcan neck-breaking technique. Sarek suffers a heart attack, and Spock is the only donor with matching blood, but when Andorian delegate Thelev stabs Kirk, Spock insists on staying in command while a mysterious vessel shadows the ship.
Kirk returns to the bridge to free Spock for the surgery and outwits the fast attacker by cutting power to look crippled, then disabling it with a surprise phaser strike. The ship destroys itself and Thelev, an Orion agent disguised as an Andorian, dies from a delayed poison. D. C. Fontana's script introduced Sarek, Amanda, the Andorians and the Tellarites, and its family conflict became a foundation for later stories about Spock.