 The Enterprise is on a largely protocolary mission: transporting diplomatic delegates to a Federation meeting on the neutral planetoid Babel, including the Vulcan ambassador Sarek and his human wife Amanda, who refuse to be shown around Mr. Spock, although they are his parents- Sarek refused any contact for 18 years because Spock chose a Starfleet career over his pacifist alternative of scientific study. The delegates already start to quarrel about the conference s subject, whether to admit the wealthy, lithium-rich Corydon system to the Federation. An unannounced small alien scout ship at warp 10 parallels the Enterpise s course; it is in contact with someone on board. Then a delegate, who just walked away angry from Sarek after hearing Vulcan approves admission, is found killed with an expertly broken neck, as a Vulcan might do. Sarek has a heart attack which is very hard to treat, except with a blood transfusion for which Spock volunteers as donor, although McCoy considers the risk to great, but pulls out when an Andurian delegate s violent attack on Kirk makes Spock acting commander, disregarding his mothers emotional plea, until Kirk resumes command still weak, and then the alien attacks, while its contact aboard is still unknown...