Sequiz

Sequiz was a planet inhabited by aggressive beings. A type of dust found on the planet would, if exposed to sufficient amounts of ultraviolet light, begin moving by itself, emit a high-pitched singing noise, and eventually form into a monstrous shape capable of attacking others.

A space probe brought back samples of dust which had originated from Sequiz; these were examined by Dr. Rossi in a UNIT lab based in Black Peak. He was very excited at the results when he exposed the dust to ultraviolet light, but the Brigadier, recognizing the potential danger, shut down the experiment. Agents of a foreign power broke into the lab, abducted Dr. Rossi and stole the sample so he could continue his experiments. The Third Doctor and the Brigadier tracked them down, but Rossi activated his ultraviolet machine to full power, enabling the dust to reach its final shape. Before it could attack Rossi, the Doctor battled the creature, forcing it to go back to Sequiz. (DWA: Caught in the Web)