Thrown back in time by a black star, the Enterprise appeared over late-1960s Earth and had to erase all evidence of its visit after beaming aboard pilot John Christopher.
The Enterprise's encounter with a high-gravity black star hurled the ship into Earth's past, near the time of the first crewed Moon launch. Detected as a UFO, it was intercepted by an Air Force jet, and Kirk had the pilot, Captain John Christopher, beamed aboard as his aircraft broke apart.
Spock discovered that Christopher's future son would lead an important mission to Saturn, so the pilot had to go home. Kirk and Sulu recovered film of the ship from an Air Force base, and a slingshot around the Sun returned Christopher to the moment before he saw the Enterprise.