Sequiz was a planet inhabited by aggressive beings. A type of dust found on the planet, if exposed to sufficient amounts of ultraviolet light, would begin moving by itself, emit a high-pitched singing noise and eventually become a monstrous shape capable of attacking others.
A space probe brought back samples of dust which had originated on Sequiz. They were examined by Dr. Rossi in a UNIT lab in Black Peak. He was very excited at the results when he exposed the dust to ultraviolet light, but the Brigadier, recognising 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. (PROSE: Caught in the Web)