The First Doctor gave Susan a new TARDIS key as a replacement for the one he destroyed. (TVThe Sensorites)

Later, knowing she would likely never see the TARDIS again, she intentionally abandoned this key in the dirt on the bank of the River Thames, after her grandfather helped her decide to start a new life in the 2160s by locking her out of the TARDIS. (TV: The Dalek Invasion of Earth)

Susan later recovered her key, and kept it with her in her new life on Earth. She knew that the Doctor's TARDIS was on its way whenever her key would glow. (AUDIO: Lucie Miller)

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