Spock's half-brother Sybok seized ambassadors on Nimbus III, then hijacked the Enterprise-A to search for the mythical planet Sha Ka Ree at the center of the galaxy.
Nimbus III had been set up as the Planet of Galactic Peace, a joint project of the Federation, Klingon Empire and Romulan Star Empire, but it had become a poor and lawless outpost. In 2287 a Vulcan named Sybok, who rejected logic, took the three powers' representatives hostage with a band of followers he called the Galactic Army of Light. The barely ready USS Enterprise-A was sent under Kirk to rescue them.
The rescue failed when Sybok's people took over the ship. Sybok revealed he was Spock's half-brother and won over much of the crew by helping them face their deepest pain. He took the ship through the Great Barrier to the planet he believed was Sha Ka Ree, the source of creation, pursued by a Klingon ship commanded by Klaa.
On the planet the crew met a being that claimed to be God but wanted only the starship to escape. Sybok gave his life to hold it back. The Klingons attacked, but the Klingon ambassador Korrd ordered Klaa to stand down, and the Klingon ship rescued Kirk. These events are shown in Star Trek V: The Final Frontier.