is sent to investigate an enormous cosmic cloud that has entered from beyond the edge of the galaxy and is approaching the planet Mantilles on the furthest edge of the Federation. Sensors indicate the cloud is 800,000 kilometers across and half that in depth, and is composed of a mixture of matter and energy. It enters the Mantilles system and approaches the outermost uninhabited planet, Alondra... and envelops it. Spock scans the area and determines that the cloud is consuming the planet. It then changes course and heads directly toward Mantilles. The Enterprise
moves to intercept it in time and Spock notes that if the creature can consume stars as well as planets, it could pose a threat to the entire galaxy. Kirk wonders if they should notify the 82 million inhabitants of Mantilles or if the resulting panic would cause more harm then good. He tells McCoy that the governor is Bob Wesley, a former Starfleet officer and a steady man, and McCoy advises him to contact Wesley and notify him of the situation.
As the Enterprise
intercepts the cloud, Spock scans its chemical composition and concludes it originated from outside the galaxy. As they approach the cloud it extends plasma tentacles and pulls the ship into itself. Kirk orders phaser fire but the cloud simply absorbs the energy. Large chunks of antimatter approach the ship and they're forced to feed antimatter into the shields to repel them. Scotty warns that the engines can't maintain the deflectors at the necessary high output. Spock analyzes the cloud and concludes that it is a living being. McCoy agrees, noting that the mist-like makeup is acting like an enzyme that will break up the ship's hull if the shields fail.
Wesley contacts Kirk and tells him that they have no time to evacuate more then a small part of the population: the children. With three hours and 20 minutes until the creature intercepts Mantilles, Spock concludes that the creature feeds on planets, breaking them up and using the resulting energy as food. The chunks of antimatter are teeth that break up matter for digestion. They locate an opening near the "top" of the creature and set course for it, passing through a central core area. They're pulled into the core area and find themselves surrounded by stalactite-like protrusions of antimatter. McCoy concludes they're in an area similar to the small intestines in a human and the stalactites are villi, which absorb nutrients from matter passing through. They watch as a piece of matter touches the villi, and explodes. The villi absorb the energy and regenerate. The engines begin to labor under the strain of maintaining the deflectors at full strength and Scotty warns that the engines will be completely drained in 21 minutes. Scotty suggests they use a piece of the antimatter to regenerate the warp reaction, and beam aboard a piece inside of a force field box. They do so and Scotty and Kirk get the antimatter to the warp nacelles and reinitiate the warp reaction with seconds to spare.
With 42 minutes remaining, Spock locates the entity's "brain" and speculates that it may be intelligent. The only action it has displayed indicating intelligence is the course change toward Mantilles, but that may have been an instinctive movement toward perceived food. Kirk orders a course change toward the brain and has Sulu prepare weapon fire to destroy the entity. Spock notes that Starfleet regulations prohibit the taking of sentient life but Kirk notes that they don't know if it's intelligent and he has to measure that against the 82 million lives on Mantilles. He wonders if they have the right to kill and McCoy observes that they have no other choice. However, Spock warns that the brain is too vast for their entire ship's weaponry to have any effect. However, they can kill the creature by self-destructing the Enterprise
. Kirk orders Scotty to prepare the self-destruct while Wesley informs them that the population of Mantilles has accepted that only the children can be saved. Kirk asks if Wesley's 11-year-old daughter Katie is there and assures Wesley that she'll be okay.
Running out of time and options, Kirk suggests that Spock determine if the entity is intelligent by using the Vulcan mind meld. Spock warns that he has to make contact to use it, but then suggests they focus the ship's sensors on the creature's synaptic relapses and run the results through the Universal Translator. Spock and Kirk go ahead and Spock reaches out with his mind. They make contact with the entity, which is surprised and puzzled to learn of creatures inside of itself, tens of thousands of times smaller but equally intelligent. Scotty arms the self-destruct and the entity starts to contact Mantilles' atmosphere. With only a minute remaining, Spock is unable to convince the entity they're real. He opens his mind fully to the entity so that they share consciousness. It wanders around the bridge in curiosity, and Kirk has Uhura bring up images of Earth and its inhabitants, identifying it as their point of origin. The cloud stops as the entity accepts them as intelligence beings that it does not wish to kill. Spock suggests that it return to its point of origin rather than encroaching into the inhabited galaxy. It reluctantly agrees despite the length of the journey and departs. The Enterprise
exits through the entity's sensory grid and Spock is left to marvel at all that he perceived while merged with the entity's mind.