Spock/Starfleet/Enterprise-A

Prior to the launch of the new Enterprise, Spock was recruited to test the new brig as he was the most "intelligent and resourceful person the designers could find." However, despite his ingenuity, he failed to escape. ()
Spock joined Kirk and McCoy for shore leave at Yosemite National Park and observed Kirk climbing El Capitan with his levitation boots. He saved Kirk's life when he slipped and fell off of the mountain, grabbing his ankle just seconds before he was killed by the impact. Later, Spock told Kirk and McCoy that before leaving the Enterprise-A, he had studied all the details involving "camping out" and roasted a marshmallow over a fire. Spock's shore leave was interrupted when Commander Uhura brought the shuttlecraft to the trio's campsite to bring them to the Enterprise as the transporters were malfunctioning.
Upon the Enterprises mission to Nimbus III to resolve a hostage situation, Kirk retained Spock as his first officer. During the Enterprise strike team's rescue mission to Paradise City on Nimbus III, Spock discovered his long-lost half-brother Sybok had been the leader of the Galactic Army of Light, the group responsible for capturing the three diplomats. Sybok hijacked the Enterprise and ordered that it be brought through the Great Barrier. During the subsequent voyage in the shuttle to the mythical , Spock tried to console his brother when they could not initially find "" on the surface. 
Shortly thereafter, the landing party encountered the supernatural being when it presented itself to them. Learning of the malevolent nature of the being calling itself "God", Sybok sacrificed his life to save Spock and his friends. Spock and McCoy were beamed up to the Enterprise through the repaired transporter, leaving Kirk alone. The transporter was severely damaged when Klaa's Bird-of-Prey fired on the Enterprise. Spock saved Kirk's life from the entity by commandeering the Bird-of-Prey with the help of General Korrd and firing on it with the Klingon ship's weapons. Later, in the Enterprise-A's observation lounge, Spock reflected on the loss of his brother. Kirk told him that he had lost a brother once, but he was fortunate in that he got him back. Returning to Earth, Spock resumed his shore leave in Yosemite with Kirk and McCoy, this time playing "Row, Row, Row Your Boat" on his Vulcan lute. ()
In 2293, Spock was chosen to be the Federation's special envoy to the "Gorkon Initiative" as it was he who opened secret talks with Chancellor Gorkon following the Praxis disaster. Spock later committed Kirk to the negotiations with the Klingon Empire. During Kirk and McCoy's subsequent trial and imprisonment for the assassination of Gorkon, Spock took command of the Enterprise and the murder investigation. He led the rescue mission of Kirk and McCoy from Rura Penthe and helped stop an assassination attempt on the Federation President. Though this mission was successful, Spock blamed himself for endangering Kirk and the consequences that followed, a guilt that lasted seventy-five years. (; )