Material deleted from the final broadcast of the episodes explains that Baltar had originally been a Colonial military officer, who led an expedition to discover new sources of tylium for mining. After discovering a particularly rich tylium deposit on Carillon, Baltar decided to go into business for himself, leaving the military. He falsified records, declaring Carillon too minimal for mining, and set himself up with the Cylons and Ovions, who were also mining Carillon.
Traitor
Towards the last years of the Thousand Yahren War between the Cylons and the Twelve Colonies of Man, Council-of-Twelve member Baltar acts as a liaison between the Twelve Colonies and the Cylons, and arranges for a peace conference that would bring an end to the war, with the Cylons apparently conceding defeat. However, unknown to the Colonials, Baltar has made a deal with the Imperious Leader of the Cylons: Baltar would conduct the peace conference to lull the Colonials into a false sense of security, while the Cylons prepared for a massive attack on the Colonial military and the Twelve Colonies. In return, the Imperious Leader promised Baltar that his home colony would be spared from the attack, and Baltar would be installed as its dictator. It is never stated which of the Twelve Colonies was Baltar's home. Since the Cylons are dedicated to the complete annihilation of humanity, the Imperious Leader goes back on his word, and destroys all twelve Colonies in the attack.
After the attack
A handful of humans flee in civilian ships, under the protection of the sole surviving battlestar, the Galactica. In the meantime, Baltar goes before the Imperious Leader aboard his flagship, enraged that the Cylons had not held up their end of the bargain. The Imperious Leader coldly responds by explaining that every human being must be exterminated, and then sentences Baltar himself to death, reasoning that any being who would see his own race destroyed could not be trusted. As originally depicted in the theatrical version, Baltar is supposedly decapitated when one of his Cylon escorts holds him fast and the other draws its sword across Batlar's throat. However, when the film was presented on television preceding the television series, the scene was reshot so that the Imperious Leader instead orders that Baltar be brought away for public execution, presumably to a prison baseship or the Cylon homeworld. However, soon thereafter, the Imperious Leader is destroyed when his basestar gets too close to the exploding planet Carillon.
Pursuer of the ''Galactica'' and its ragtag fleet
The succeeding Imperious Leader spares Baltar's life, believing that Baltar, being human, would have a superior insight into the minds of the remnants of humanity which the Cylons are pursuing. To this end, the Imperious Leader installs Baltar as the commander of a Cylon basestar, with an "I.L.-series" Cylon named Lucifer as Baltar's second in command; the two other basestars in Baltar's taskforce are also subservient to him. Baltar then makes it a personal quest to vanquish his rival, Commander Adama, and destroy the Galactica and its ragtag fleet.
Freedom
In the final episode of the series' original run, "The Hand of God", Baltar makes a deal with Adama. Baltar provides the Colonials with technical information on Cylon basestars, which Apollo and Starbuck use to render a wayward Cylon basestar "blind" to the Galactica, which then destroys it. In exchange, Adama agrees to "maroon" Baltar with sufficient equipment and supplies to allow him to live on the first habitable planet that the Fleet passes on its journey; Adama also reluctantly gives Baltar equipment for "short-range communications" so that he has "some hope of eventual rescue".
Revival attempt
In 1999, John Colicos reprised his role as Baltar for Richard Hatch's attempt to revive the series. In a short film entitled , he reveals that the Colonial fleet is still in danger after a new breed of even more dangerous Cylons emerges after a civil war.
Differences in other versions
In the novelization
tells a different story, where Baltar is a rare items dealer who had grown wealthy from his business dealings, and whose title of Count had been self-proclaimed. Later novelizations of subsequent episodes revealed that Baltar's resentment of Adama had started when they had attended the Colonial Academy together, with Adama besting him in everything.
In comics
In the Marvel Comics run from 1978–80, Baltar was portrayed more as a darker skinned human, and was executed by the Cylons as seen in the treasury sized edition. In the 1990s, Baltar appears in the Maximum Press comic book series, still pursuing the Galactica, after having been freed by Commander Adama in exchange for bartering his knowledge of the Cylons. Dynamite Comics expands on this concept in a 2014 story in which Baltar is found on the planet abandoned by Adama. He remembers his life to this point and how he felt betrayed by the Colonies and thus sided with the Cylon empire, only to be betrayed himself by the Cylons and suddenly believes he himself has been transmuted by the Cylons into a cybernetic being; throughout the story he deals with one particular First Centurion, as a youth, before the massacre of the Colonies, and when he is finally rescued.
In the feature film version, when Baltar meets the Imperious Leader, and it is explained to him that every human being must be exterminated, and Baltar is sentenced to death, Cylon Centurions immediately execute Baltar on the spot by beheading him. This differs from the television version, where his execution is delayed and he is sent to be prepared for public execution—with this Imperious Leader being killed at Carillon and, in the epilogue made for the movie's television broadcast, the succeeding Imperious Leader sparing Baltar's life and announcing that he would be provided with a basestar and Lucifer as a liaison—thus setting him up for his role in the series.
In the re-imagined 2003 version
In the 2003 miniseries and 2004 series, the character is reimagined as Gaius Baltar, a scientist who is seduced and tricked by a human-looking Cylon into sabotaging the Colonial security systems, allowing them to attack the Colonies. He successfully covers up his role in the attacks, and using his reputation as a scientific genius, positions himself as part of the leadership of the survivors, serving as vice president, president, a puppet dictator under a Cylon occupation force, and a refugee aboard a Cylon ship following that administration's collapse. He is mentally unstable, a condition which is exacerbated by his interactions with a manifestation of his Cylon lover which only he perceives.
