The senior guard was the head of the guards who was loyal to the Daleks in an alternative 22nd century Earth. After overhearing the Controller talking to the Third Doctor and Jo Grant about preventing the Dalek invasion of Earth and the Controller betraying the Daleks, he informed on the Controller to his master. When the Daleks exterminated the Controller, the Senior Guard was made his replacement. (TV: Day of the Daleks)
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