Having found a Federation colony of 100 people completely destroyed, Kirk and the Enterprise have to deal with a nearby Klingon vessel which they believe must be responsible for the colony's destruction. When the Klingon ship is disabled, they, in turn, assume they were attacked by the Enterprise. There is obvious tension between the Enterprise crew and its Klingon enemies. Unbeknown to Kirk and his Klingon counterpart, Kang, this is the work of an alien being that gets its energy from the friction and emotions between sentient beings. The natural animosities between the two parties feed its appetites. When the creature is beamed aboard the Enterprise, it purposely creates tension among the crew, to its benefit. The situation eventually forces Kirk and Kang to work together to defeat it. Written by
Did You Know?
Although intra-ship beaming is routine in later incarnations of 'Star Trek' (where it is called "site-to-site transport"), this is the first and only time it is done in the original series, although it is also referenced in "Shore Leave" when Sulu notes that Spock is beaming down "from the bridge". See more
Just before Chekov forces a kiss on Mara, we see in the close-up shots of her that she is wearing lipstick. Kirk and Spock then arrive and intervene, and in the cutaway shots of Mara during Kirk's subsequent speech, the lipstick is gone. Then Kirk has Spock take her, and as she and Spock exit frame, we see she's wearing lipstick again. See more
Klingons claim to have honored the truce, but there've been incidents, raids on our outposts...
Captain James T. Kirk
No proof that the Klingons committed it.
What proof do we need? We know what a Klingon is.