- For the mirror universe counterpart, please see Gardner (mirror).
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In 2143, Gardner was a pilot in the NX test program. Along with Commanders Duvall, A.G. Robinson, and Jonathan Archer, he competed for the position as pilot in the effort to break the warp two barrier in the NX-Alpha. Ultimately, however, Robinson was chosen to conduct the flight. (ENT: "First Flight")
By 2151, Gardner had been promoted to captain, and was endorsed by Vulcan ambassador Soval for the command position of the Enterprise NX-01, Earth's first warp five starship. Eventually the candidates were narrowed down to Archer and Robinson, with the posting ultimately going to Archer. Soval, however, continued to voice his objection to Archer's selection, feeling he was too impulsive for such an important mission. (ENT: "Shadows of P'Jem", "First Flight")
By 2154, Gardner had been promoted to admiral, and replaced Admiral Maxwell Forrest in overseeing Enterprise's mission after Forrest was killed in a bombing of the United Earth Embassy on Vulcan. In that capacity, he ordered Enterprise to return to Earth subsequent to a communication with Administrator V'Las, who was attempting to remove the Enterprise crew as witnesses to a planned massacre of the Syrranite movement. Charles Tucker, in command of Enterprise at the time, ultimately chose to disobey this order, however, when he learned of a planned Vulcan invasion of Andoria, and subsequently ordered Enterprise to Andorian space in an attempt to warn them. Later, Tucker contacted Gardner, informing him that Enterprise had taken up position with an Andorian fleet to combat the invasion force, in hopes that V'Las would halt his attack, rather than let it be known that Vulcan forces were responsible for Enterprise's destruction in the subsequent battle. (ENT: "Awakening")
Later that year, when Archer came to believe that he was going to die due to infection by a silicon-based virus, he warned Commander T'Pol, his first officer, to attempt to remain in command of Enterprise, and to oppose Gardner if he attempted to assign a new captain. (ENT: "Observer Effect")
Also in 2154, Gardner granted Archer's request of the redeployment of every available Starfleet vessel to Enterprise's location in an attempt to complete a sensor grid with which to detect and locate a vessel which had been attacking various ships in the area. However, as the Starfleet vessels would have taken a matter of weeks to arrive, Archer was forced to rely on Vulcan, Andorian, and Tellarite assistance to complete the grid. (ENT: "United")
Later that year, Gardner granted Archer's request to embark on a mission to rescue Doctor Phlox, who had been abducted by the Klingons in an attempt to force Phlox to assist them in creating Klingon Augments. (ENT: "Affliction")
In the script for the undeveloped Romulan War film project Star Trek: The Beginning, Gardner's daughter, Penelope, was the love interest, and eventual wife, of the main protagonist, Tiberius Chase, making him a direct ancestor (presumably great-grandfather) of James T. Kirk. 
Gardner was named by "Shadows of P'Jem" co-writer Phyllis Strong in remembrance of a friend who died at the World Trade Center during the terrorist attacks on September 11, 2001. (Information provided by Mike Sussman)