Doctor Who: Fury from the Deep (1968)

The TARDIS brings the Doctor, Jamie and Victoria to the present-day North Sea coast. Something nasty is lurking in the gas pipelines of the North Sea, and before long the nearby refinery is under attack. The Doctor discovers that a form of living seaweed is at work, and that a number of the base personnel have been possessed. Can he and his friends work in time to prevent an entire take-over of Earth?