A large cocoon-like craft heads towards Earth and disgorges smaller, similar craft. The caterpillar-like aliens attack a meeting of the planet's scientists and control them with rays emanating from the single eye in their heads. The TARDIS materialises in a thick jungle and the Doctor is captured by the aliens. He is taken to a dome made of a substance that looks like silk but is as tough as steel. Inside, he is imprisoned with the scientists. John and Gillian find the offices of World Pest Control. The aliens are rendered helpless with a spray administered from a helicopter.