Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Writer: D. C. Fontana
Dir.: Joseph Pevney

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Christine Chapel: Majel Barrett

Sarek: Mark Lenard
Gav: John Wheeler
Thelev: William O'Connell
Shras: Reggie Nalder

Stardate: 3842.3

The Enterprise is bringing guests to an important conference on Federation level. They are brought to Babel, a planet code - named for this event. There is not always mutual consent to be taken granted since the delegates come from various worlds. The Vulcan ambassador, Sarek, and his wife, Amanda, a Human, are welcomed on board. Doctor McCoy has Spock teaching him the Vulcan salute, but when Sarek steps past Spock, ignoring his greeting, the doctor withdraws his hand. When Kirk offers Spock to beam down to Vulcan to visit his parents, Spock informs him (to Kirk's embarrassment) that ambassador Sarek and Amanda are his parents. 

Ambassador Sarek and his wife are my parents

When Kirk suggests that Spock gives the Ambassador a ship tour, Sarek icily asks for another guide and leaves with his wife, both gently touching each other's closed pointing and middle-fingers. Amanda will later explain to the bewildered captain that Sarek has not spoken with Spock for eighteen years, because he didn't approve Spock's choice to join Starfleet. 

During a formal dinner, it appears obvious that Gav, the Tellarite Ambassador, and Sarek do not share the same interests. The Tellarite insists that Sarek states how he will vote, Sarek refuses, saying that the matter must be settled where it belongs - at the conferebnce. The Tellarite and his Andorian friend, Thelev, become angry. Both even seem to be hostile. A ship following the Enterprise adds outer jeopardy to the inner tensions on board. It is not to be identified and poses a continuous threat to the safety of the delegates. In a private conversation with McCoy and Kirk Amanda mentions Spock's pet: A sehlat. When McCoy asks her, she compares it to a huge Teddy bear. Seeing McCoy’s amusement, Sarek interrupts the conversation. He does not want his son to be ridiculed at, in spite of his rejection, but does not mention the reason for his behavior. Spock explains that a sehlat has six-inch fangs. 

Suddenly Gav is found dead. The previous incidents indicate Sarek to be the most suspected person, though Spock claims his father had no logical purpose to kill the Tellarite ambassador and Vulcans do not kill without a logical motive. Sarek suffers from a heart attack and his condition appears to be severe. He needs immediate surgery, but McCoy hesitates. He doesn't feel selfassured enough about Vulcan anatomy and physiology. The operation would require a great amount of Vulcan blood and Spock is the only available donor. Spock finds a drug which can improve his blood cells reproduction and suggests to use it so that he can give the requested amount of blood, without being hurt, but McCoy is not convinced.  

In these moments Thelev tries to kill Kirk. The captain survives the stabbing, but – heavily wounded – cannot leave sick bay. Sarek's condition worsens and McCoy finally makes up his mind, but now Spock is in command and cannot leave the bridge. Not even his mother's pleas can change his mind and Amanda is deeply upset. McCoy is now aware that the operation has to be done, without losing any further time. He helps Kirk to pretend to have recovered. Kirk makes it back to the bridge and insists in assuming command. Giving in, Spock leaves the bridge and is now free to donate the urgently needed blood.

The ship following the Enterprise is by now identified as Orion, and so is Thelev, who has been disguised as Andorian. The real murderer is found. The Orion ship attacks the Enterprise and is destroyed. The murder kills himself to avoid questioning, while dying he tells Kirk that he and his companions on the Orion ship were bond to a suicide mission. Father and son – on speaking terms again after the successful operation – make Amanda realize that there is unspoken understanding between them, as she complains about the frequent use of the word "logic". Trying to use the situation of having the captain and the two Vulcans as patients, McCoy succeeds in having, for once, the last word.

Why did you marry her?