Captain Braxton was a Starfleet officer from the 29th century. In that capacity, he piloted the Federation timeship Aeon, and commanded the USS Relativity.
Braxton was first encountered by the USS Voyager in 2373, when the Aeon arrived in the Delta Quadrant through a temporal rift. Braxton claimed that Voyager had been responsible for the destruction of the Sol system in the 29th century, and had traveled back to prevent that occurrence by destroying Voyager. The Voyager crew was able to counter Braxton's attack, however, destabilizing his temporal rift, into which both ships were pulled. Both ships were subsequently sent back to Earth, the Aeon to 1967, and Voyager to 1996.
When Voyager arrived in Earth orbit in 1996, they discovered Braxton living homeless on the streets of Los Angeles. Poor treatment at the hands of the 20th century authorities had left him paranoid and aggressive, blaming Voyager for his predicament. Braxton informed Captain Janeway of his deduction that, if Starling were to use the timeship to travel to the 29th century without properly recalibrating the ship's temporal matrix, then that could have been the cause of the temporal explosion which destroyed Earth's solar system. The crew of Voyager was able to prevent this future by destroying the Aeon with a manually-launched photon torpedo just before Starling entered the temporal rift, resulting in the creation of a new timeline in which the explosion never occurred. Shortly afterwards, Voyager encountered another temporal rift, created by Captain Braxton's counterpart from the new timeline. Having detected Voyager's anomalous presence in the 20th century, Braxton had traveled back in time to escort Voyager back to their proper place and time. Despite Janeway's plea to return them to Earth, as Voyager had been stranded in the Delta Quadrant for two years, Braxton insisted on upholding the Temporal Prime Directive.