Tony Mitchell (The Sea Devils)

Tony Mitchell was an officer in the Royal Navy. (PROSE: Doctor Who and the Sea-Devils) He served as Commander Ridgeway's second-in-command on the submarine expedition to search for the Sea Devils. They captured him and the rest of the crew, but the Third Doctor helped free them.

By one account, Mitchell's rank in the Navy was Lieutenant Commander; (TV: The Sea Devils) by another, he was a Sub-Lieutenant. (PROSE: Doctor Who and the Sea-Devils)