This episode begins with Captain Kirk trying to convince the leader of the planet Halkan to provide The United Federation of Planets with the rights to mine dilithium on the planet. The Halkan leader informs Kirk that the Halkan government has refused to allow The Federation to have mining rights. The Halkans are pacifists and they believe that while The Federation is not currently engaged in any military conflicts The Federation cannot provide a guarantee that they will always remain at peace. During their conversation with Kirk, the Halkans mention that they are willing to die for their beliefs, although Kirk assures them they will not have to:
Our dilithium crystals represent awesome power. Wrongful use of that power even to the extent of the taking of one life would violate our history of total peace. To prevent that, we would die, Captain, as a race if necessary—Halkan Leader
As Kirk and the away team are beaming back onboard the Enterprise they are transported to a parallel universe controlled by The Empire, which is for all intents and purposes the evil twin of The Federation. Empire Spock greets the away team and asks if the Halkans have agreed to the mining rights. Kirk tells him they have not and Spock then mentions that the Halkans “have chosen suicide,” indicating that The Empire will take the dilithium by force. The Halkans are still pacifists in this alternate universe and again refuse to provide the mining rights, and although Kirk is able to return to his own universe, the audience can safely assume that The Empire destroyed the Halkans in the alternate universe.
Like Mudd’s Women, Mirror, Mirror exposes the common thought that life is an acceptable currency for an energy resource. The Halkans realize the great power that energy resources can provide and that sometimes that power is abused and results in the taking of life. Therefore, they offer up their own lives in exchange for the lives that may be taken as a result of that abuse of power, with the hope that The Federation and The Empire will also see the link between energy resources and death and will be unwilling to pay that price. The Federation refuses the exchange but The Empire does not as they value the power and comfort that the energy resource provides them more than the lives of the Halkans. This equivalency of life and energy by the stronger force is a defining characteristic of a negative energy narrative.