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 when his wife, Maggie, falls mysteriously ill. Van Lutyens and the Chief Engineer attempt to find out what is blocking the impeller.
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.