The crew of the newly commissioned USS Enterprise (NCC-1701-A) are enjoying shore leave after the star ship's shakedown cruise goes poorly. At Yosemite National Park James T. Kirk (William Shatner), recently demoted back to Captain after the events of the previous two films, is camping with Spock (Leonard Nimoy) and Dr. Leonard McCoy (DeForest Kelley). Their leave is interrupted when the Enterprise is ordered by Starfleet Command to rescue human, Klingon, and Romulan hostages on the planet Nimbus III. Learning of the Enterprise's mission, the Klingon Captain Klaa decides to pursue Kirk for personal glory. Other officers include Leonard McCoy (DeForest Kelley), Medical officer, Montgomery Scott (James Doohan) engineer, helmsman Hikaru Sulu (George Takei), Pavel Chekov (Walter Koenig), Uhura (Nichelle Nichols)Enterprise reaches Nimbus III, but its transporter beams are inactive, and the Klingon vessel is just 2 hours away and is approaching rapidly. Kirk decides to take a team to the surface to negotiate. Kirk and his landing party attack the hostage location, but the hostages instead take Kirk as captive & force Kirk's team to surrender.On Nimbus III, the Enterprise crew discovers that renegade Vulcan Sybok (Laurence Luckinbill), Spock's half-brother, is behind the hostage crisis. Sybok reveals the hostage situation was a ruse to lure a star ship to Nimbus III. Sybok wants to use a ship to reach the mythical planet Sha Ka Ree, the place where creation began; the planet lies behind a seemingly impenetrable barrier near the center of the galaxy. Sybok uses his unique ability to reveal and heal the innermost pain of a person through the mind meld to subvert the wills of the hostages and crew members. Only Spock (Spock's greatest fear was his father's nonacceptance of Spock as a Vulcan) and Kirk prove resistant to Sybok; Spock is unmoved by the experience and Kirk refuses the Vulcan's offer, telling him that his pain is what makes him human. For example McCoy''s greatest fear was that McCoy assisted his father in medical euthanasia, and could not save him, despite all the knowledge he had at his disposal. Sybok reluctantly declares a truce with Kirk, realizing he needs his leadership experience to navigate the Enterprise to Sha Ka Ree. As Sybok, Kirk and rest of the crew get near the Enterprise, Klaa's vessel reaches Nimus III as well. Kirk pleads Sybok to assume command so he can guide the enterprise out of this. Kirk instructs Scotty to lower shields just long enough for Sulu to fly the shuttle into the Enterprise manually at full speed. As soon as the shuttle is in, Chekov takes it to warp speed, before Klaa's ship can fire upon its position.The Enterprise successfully breaches the barrier, pursued by Klaa's vessel, and discovers a lone blue planet. Sybok, Kirk, Spock, and McCoy journey to the surface, where Sybok calls out to his perceived vision of God. An entity appears, and when told of how Sybok breached the barrier, demands that the star ship be brought closer to the planet. When a skeptical Kirk inquires, "What does God need with a star ship?", the entity attacks him in retribution. The others doubt a god who would inflict harm on people for pleasure. Meanwhile, unnoticed in the commotion, Klaa's spaceship also penetrates the barrier and cloaks itself and starts to monitor the situation.Realizing his foolishness, Sybok sacrifices himself in an effort to combat the creature and allow the others to escape. Intent on stopping the being, Kirk orders the Enterprise to fire a photon torpedo at their location, to little effect. Spock and McCoy are beamed back to the ship, but Klaa's vessel attacks the Enterprise before Kirk can be transported aboard. The vengeful entity reappears and tries to kill Kirk when Klaa's vessel destroys it in a hail of fire (Spock and the Klingon General Korrd force Klaa to stand down). Kirk is beamed aboard the Klingon ship. The Enterprise and Klingon crews celebrate a new detente, and Kirk, Spock, and McCoy resume their vacation at Yosemite.