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.
While the Doctor begins his investigation into the weed creatures back in the Tardis, Robson is attacked at the refinery.
Van Lutyens descends into the impeller shaft and isn't heard from again. The Doctor and Jamie decide to investigate.
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 an infected Robson and realise they can destroy the Weed Creature with sound waves.