Flast was a Cryon who had been captured and imprisoned by the Cybermen. She told the Sixth Doctor about the Cybermen's plans to change history by diverting Halley's Comet so it would destroy Earth.

The Doctor gave her his sonic lance, which she used to slowly heat the stored vastial. The Cybermen killed her by exposing her to above-zero temperatures and she was boiled alive, but the lance ignited the vastial, triggering an explosion which destroyed Cyber Control. (TV: Attack of the Cybermen)

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