Calderon IV was a planet the Third Doctor and Sarah Jane Smith visited. On Calderon IV, they met the android maker, whom they defeated in an hour and a half. The Doctor left a signal device behind in case they were needed again.
Five years later, the android maker made a perfect replica of the Doctor. He programmed it to murder and replace the Doctor so he could have an audience with Praesidium and assassinate him with an explosive in his head. However, the android became useless when it turned out the Doctor had regenerated. (PROSE: The Android Maker of Calderon IV)