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 tranquilliser 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 Second 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…