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Major Jackson "Dipper" Spencer is the CAG aboard Galactica just before her scheduled decommissioning and conversion into a museum. He is a liked and respected officer who mixes easily with his pilots. Off-duty he enjoys joining his pilots in a card game, while on-duty he takes his work very seriously and treats his pilots with a firm but fair hand.
During the decommissioning ceremonies for Galactica, Spencer plays second fiddle to Captain Lee Adama, allowing the younger man to fly the N7242C newly-restored Viper Mark II—the vehicle flown by Lee's father, the then Lieutenant William Adama during the First Cylon War.
Following the decommissioning ceremony, Spencer leads his squadron—the last fully-operational squadron on Galactica—to Caprica, presumably for later redeployment in the Colonial Fleet. While en route, the Cylons launch their surprise attack on the Twelve Colonies.
Alerted to the attack by Galactica, Spencer locates two Cylon Raider fighters and, after ordering the accompanying Raptor 312 reconnaissance vehicle away from the combat zone, orders his squadron to weapons free to intercept and destroy the Raiders. However, as the targets approach, all Vipers in Spencer's squadron suffer simultaneous failures in their flight control, communications and weapons systems as the Cylons infiltrate their ship computer using the backdoor programming of the Command Navigation Program.