The removal of Spock's brain in 2268 by Kara, leader of the Eymorg of Sigma Draconis VI, who needed it to run the machinery that sustained her underground people.
A woman beamed aboard the USS Enterprise, rendered the crew unconscious and left. When they woke, Spock lay in sickbay with his brain surgically removed. McCoy could keep the body alive for only a day.
In the Original Series episode Spock's Brain, Kirk follows an ion trail to the Sigma Draconis system and its frozen sixth planet. Men, the Morg, live as primitives on the surface, while the women, the Eymorg, dwell below in comfort provided by a machine they no longer understand. Its Controller had failed after ten thousand years, and Spock's brain had been installed in its place.
McCoy uses a device called the Teacher to gain the surgical knowledge to restore the brain. As it fades, Spock, able to speak again, talks him through the last steps.