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The Guardian basestar was constructed by the Cylons at the end of the first Cylon War and is a part of the evolution towards modern basestar design. The first Hybrid to be developed was integrated into the systems of this ship.
During Operation Raptor Talon, Lieutenant William Adama was shot down over an ice planet and encountered a facility where the Cylons were experimenting on humans in several ways, eventually creating the first Hybrid. Unable to free surviving captives as the facility began to crumble around him, Adama witnessed the launch of the Guardian basestar. Approximately 40 years later, the vessel is once again encountered by the Colonial military.
While searching for a missing Raptor with a civilian science team aboard, Starbuck and Showboat encounter a squadron of Raiders identical to the models used in the Cylon War. Outnumbered by the enemy, the two pilots retreat towards the safety of Pegasus. Both manage to land while Pegasus uses its point defense batteries to halt the approaching Raiders. One Raider penetrates these defenses and follows Starbuck and Showboat into the landing bay. A quick-thinking Starbuck shoots the craft down, leaving the Raider sufficiently intact for observation.
Within the shot-down Raider, the Colonials find a trio of old-style Centurions. Upon viewing their remains, Sharon Agathon recalls the legend of the Guardians; a faction of Centurions that broke away from the core group when they were to be replaced with newer, non-sentient models. They were given the name 'Guardians' because the baseship they escaped with carried the first Hybrid, a figure that these Centurions worshiped like a deity.
After learning of the Guardians from Agathon, and connecting the information with his experiences 40 years ago, Admiral Adama realizes that the missing Raptor crew is likely on the Guardian basestar, and that they are undergoing the same treatment as the civilians on the ice planet were 40 years ago. Unwilling to leave these crewmen to be mutilated, Adama orders an attack on the basestar. Having learned the vessel's likely location from the Raider's navigational computer, Pegasus jumps into the vicinity of the ship, and instigates the Battle of the Guardian basestar. The Pegasus seems to be a match for it by itself and it is ultimatly destroyed by Major Kendra Shaw who detonates a nuke on board after talking with the First Hybrid.