(covers information from several alternate timelines)
Captain John Christopher was a male Human military officer in the 20th century. He served in the Air Defense Command, a command in the United States Air Force. He had qualified as an expert pilot in this command. His serial number was 4857932. As of 1969, he was a husband and the father of two daughters. After 1969, he was the father of Shaun Geoffrey Christopher, the commander of the first successful mission to Saturn.
In 1969, the USS Enterprise arrived in low Earth orbit from the year 2267, through an accidental time warp. Captain Christopher was scrambled in his F-104 Starfighter to intercept the Enterprise, which the Air Force had detected as a UFO. Detecting the approach of Christopher's jet, and fearing the use of nuclear warheads against them, Captain Kirk ordered the jet held in a tractor beam. But the metal of which the jet was constructed proved to be too fragile to withstand the force of the tractor beam, and it began to break apart, forcing Kirk to order Christopher beamed aboard.
Christopher's arrival aboard the Enterprise presented the crew with an interesting problem: Christopher could not be returned to Earth, as his newly acquired knowledge of the future might lead him to alter history inadvertently. With no choice, Kirk informed Christopher that they could not return him, although Christopher refused to accept this. Christopher tried to escape, pointing a phaser at transporter chief Kyle, demanding to be beamed to the surface, but Kirk arrived in time and knocked him out.
While Christopher was recuperating in the sickbay, Spock informed Kirk and Christopher that he had uncovered historical evidence that indicated Christopher had to be returned to Earth; that although Christopher himself had made no relevant contribution to history, his future son, Shaun Geoffrey Christopher, would lead the first Earth-Saturn probe in the early 21st century, and that without Christopher on Earth to father this son, the mission would never take place.
Later, Kirk and Lieutenant Sulu beamed down to the Air Force base to covertly retrieve evidence of the Enterprise's presence in 1969, but Kirk was captured in the process. Christopher offered to help Spock retrieve Kirk, but only on the condition that Christopher be allowed to accompany the rescue party. Spock agreed, and the rescue of Kirk was successful, but during their time on the base Christopher stole a firearm, with the intention of staying and reporting what he knew. Before Christopher was able to do so, however, Spock incapacitated him with a Vulcan nerve pinch.
Spock and Chief Engineer Montgomery Scott devised a plan to return Christopher without any damage to the timeline: they would recreate the time warp via a slingshot effect around the Sun, returning them to their own time. Since, as they gathered speed approaching the Sun, they would initially travel back in time, they would simply return Christopher to a point in time before his transport aboard the Enterprise. Their plan worked, and Christopher was returned to the cockpit of his jet before he was beamed aboard, leaving him with no memory of what had transpired--thereby causing the events he had experienced to "unhappen." (TOS: "Tomorrow is Yesterday")
Of note is that when Christopher was given a Starfleet uniform to wear, its command division tunic bore single full braided metallic-gold stripes ringing its sleeves, indicating the rank of lieutenant. The USAF rank of "captain" is, in fact, equivalent to the US Naval rank of "lieutenant." Both are O-3 officer grades in the United States armed forces.
The Air Force service number Captain Christopher provided (4 857 932) was part of an unused range of service numbers (four million through seven million) created by the Air Force after World War II but never distributed to service members. The range of Air Force officer numbers in use during 1969 would have had Captain Christopher either holding a three million reserve officer number or, if Regular Air Force, a number somewhere between sixty and eighty thousand. (Source: National Personnel Records Center)