Pescaton invasion

In the late 20th century, the Pescatons invaded Earth, attacking and killing many people in London. The Fourth Doctor and Sarah Jane Smith discovered their weakness to high-pitched noise and set a trap for the Pescaton leader, Zor, killing him. When he died, all the other Pescatons followed suit. (AUDIO: )

Background
The Fourth Doctor first encountered the Pescatons on their dying home world in the 15th century. Their leader, Zor, had brought him there to help them find a new planet. The Doctor refused because of their barbaric qualities, defeated Zor in mental combat, and escaped back to the TARDIS. (AUDIO: )