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Miss Mackenzie was a maid and a nurse in the employ of Alan Campbell, who was actually the Master, in Scotland in 1830. She cared for Mr and Mrs Campbell, actually Aeroliths whose life forces were being extracted by the Master. When the Seventh Doctor and Ace arrived, she became more suspicious about her employer until he finally revealed himself as the Master.

After they released the Aeroliths, Mackenzie took charge of the house, and bid the Doctor and Ace goodbye. (COMIC: Cat and Mouse)