Star Trek: The Original Series, Season 3

[having heard of Kirk's alibi for the Enterprise crossing the Neutral Zone, the Romulan Bird of Prey's female commander meets Spock]

Romulan Commander:
I must admit some surprise on seeing you, Spock. We were not aware of Vulcans aboard the Enterprise.

Spock:
Starfleet is not in the habit of informing Romulans of its ships' personnel.

Romulan Commander:
Quite so. Yet there are certain ships, certain officers, that are known to us. Your situation appears most interesting.

Kirk:
What earns Spock your special interest?

Romulan Commander:
He is a Vulcan. Our forebears had the same roots and origins. Something you wouldn't understand, Captain. We can appreciate the Vulcans, our distant brothers. I have heard of Vulcan integrity and personal honour. There's a well-known saying, or is it a myth, that Vulcans are incapable of lying?

Spock:
It is no myth.

Romulan Commander:
Then tell me truthfully now, by your honour as a Vulcan, what was your mission?

Spock:
I reserve the privilege of speaking when it will not violate my honor as a Vulcan.