Bill Filer was an American agent of UNIT. He served as UNIT New York's Intelligence officer, cooperating with the UNIT UK branch. (PROSE: The Devil Goblins from Neptune)

"Washington HQ" sent him to the UK to discuss the threat of the Master. Shut out of the rest of the proceedings by Horatio Chinn, Bill left, intending to return to his base of operations. On his way back he passed by Axos. He was captured by the ship, scanned, duplicated and held prisoner alongside the Master.

His shouts for help were heard by Jo Grant when she and the Third Doctor visited the ship and got lost inside it. With the Master's help, he escaped by shooting a nerve centre. He arrived at the complex just in time to help destroy the duplicate of himself. For the remainder of the crisis, he helped the Doctor and UNIT defeat the Axons and the Master and was witness to the Nuton Power Complex explosion. (TV: The Claws of Axos)

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