An android of the Fourth Doctor was created by the Kraals after the Time Lord had been processed in a pattern analyser. The android travelled to Earth in a retro tube aboard Guy Crayford's XK-5 space freighter.
It confronted Sarah Jane Smith by the Doctor's TARDIS. She initially took it for the real Doctor, but when it told her it didn't want the Space Defence Station warned about the coming invasion, and when it was unperturbed by the emergence of an android of Sarah, she realised and fled.
The android went to the station where the real Doctor was. It found him in Colonel Faraday's office, who, along with Harry Sullivan, had been swapped for androids. The Doctor jumped out a window and the android fired after him with a gun. When Grierson attempted to activate the radar dishes, it shot and injured him. The dishes had been realigned on the Doctor's instructions to point towards the station so they would deactivate the android when turned on. The Doctor and his android fought, with the real Time Lord managing to activate the radar dishes as his counterpart raised a chair above his head to strike him. The android was frozen in place.
The Doctor reprogrammed it and had it attack Styggron, Chief Scientist of the Kraals. It threw him onto a container with the Kraal virus within, killing him. The scientist fired at the android, destroying it, but died without knowing it wasn't the real Doctor. Sarah, who ran to its body, believing it to be her friend, was told by the Doctor not to "waste any tears on him ... [he was] only an android". The android Doctor subsequently changed back to its original electronic skeleton. (TV: The Android Invasion)