Fury from the Deep

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS lands on the surface of the sea, just off the east coast of England. The time travellers use a rubber dinghy to get ashore, where they are shot with tranquiliser darts and taken prisoner by security guards as they have arrived in the restricted area of a gas refinery.

At the refinery base, run by a man named Robson, the Doctor learns that there have been a number of unexplained problems with the pressure in the feed pipes from the offshore drilling rigs. It is later revealed that one of the rigs has sucked up a parasitic form of seaweed, which is capable of releasing poisonous gas or a strange kind of foam that allows it to take control of the minds of those it touches.

The weed spreads rapidly and seems set on establishing a huge colony centred around the rigs. The Doctor makes the chance discovery that it is very susceptible to high pitched noise; consequently he is able to use the amplified sound of Victoria's screams to destroy it. Victoria elects to stay with the family of one of the refinery workers, Harris.

The Doctor, although sharing Jamie's sadness at her departure, understands her decision to settle down to a quieter life.

5 out of 83 (6.0%) raters say this story requires a previous story.