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.