The Doctor, Jamie and Victoria are taken prisoner by the staff of a gas refinery, who have lost contact with their drilling rigs and experienced mysterious drops in pressure.

Harris asks for the Doctor's help with Maggie, while Van Lutyens and the Chief Engineer attempt to find out what is blocking the impeller.

While the Doctor begins his investigation into the weed creatures, Robson is attacked by one at the refinery.

Van Lutyens' investigation of the base of the impeller places the Doctor and Jamie in danger from the Weed Creature.

With the Weed Creature poised to burst out of the pipeline and attack the base, an encounter with Oak and Quill gives the Doctor a vital clue about its weakness.

The Doctor and Jamie rescue Victoria from Robson and realise they can destroy the Weed Creature with sound waves.