OFFICIAL SYNOPSISThe U.S.S. Enterprise crew learns that a massive, planet-destroying extragalactic entity has entered Federation space and threatens every planet in the galaxy. The starship intercepts the gaseous form in an inhabited planetary sector. Almost immediately the cloud envelops the planet Alondra, breaking it into small pieces in seconds. The cloud proceeds on a direct course for planet Mantilles, inhabited by nearly 100 million people. Due to its proximity to the cloud, the Enterprise is pulled inside and is threatened by an antimatter barrage.
The ship's shields barely hold together while Spock discovers that the cloud is a living entity with a brain. He also discovers that they can escape through the top of the entity if they act quickly.
Enterprise travels through the creature's body while
antimatter continues to drain the ship's power. Scotty,
however, is able to create a power regeneration system by
using part of the entity's body. Kirk realizes that the only
way to save the planet may be to kill the creature by
exploding the Enterprise while inside. Instead, Spock
mind-melds with the cloud and allows the creature to see
glimpses of human life. The creature comes to understand
that its food — planets — contains living creatures. The
cloud allows the U.S.S. Enterprise to leave
peacefully and agrees to depart the galaxy.