Arriving on the Beach

The TARDIS materialises on the surface of the sea, just off the east coast of England. To reach the nearby shore The Doctor, Jamie McCrimmon and Victoria Waterfield use a rubber dinghy. As they explore the beach The Doctor spots a pipeline running up the beach – which he has to take a closer look at. The Doctor believes he can hear a strange beating sound coming from the pipe and so starts to loosen the screws on an inspection plate using, when Jamie asks, a sonic screwdriver. Suddenly, without any warning, the three travellers are shot with tranquilliser darts.

Shortly afterwards they awake to find themselves in a large control room inside a North Sea Gas refinery. The controller, John Robson, demands that they explain why they were tampering with the pipeline. Unsatisfied with The Doctor’s reply, and more concerned by the loss of contact with one of the drilling rigs at sea, he has his deputy Frank Harris lock them up in one of the cabins.

With the intruders out of the way, Robson can concentrate on more pressing problems. But Harris has other ideas when he asks Robson to consider shutting down the pipeline as he has gathered considerable evidence about the pressure variations that have been occurring. Robson though demands evidence prompting Harris to contact his wife, Maggie, to bring the paperwork that he had accidentally left in their living quarters. However, when Maggie eventually locates the papers she is stung by a clump of seaweed that, for some unknown reason, is with them. In disgust she throws the seaweed out onto the balcony.

Inspecting the Pipeline

Meanwhile The Doctor, Jamie and Victoria manage to escape from their makeshift cell. The Doctor decides that he and Jamie should investigate the impeller shaft, whilst Victoria should stay in the cabin. Victoria though finds herself trapped in an oxygen supply room with foam poring out of a ventilation duct. To Victoria’s horror amongst the foam is a moving mass of seaweed. On hearing Victoria's scream The Doctor and Jamie race to rescue their friend but when they arrive the seaweed has already retreated leaving only a toxic gas in the air. Back in the Harris’ living quarters Maggie has become very drowsy. She does though manage to tell her husband about the seaweed that stung her. Harris decides to fetch The Doctor. However, as soon as her husband has left Maggie rises from the bed and opens the door to the balcony where the seaweed is growing amongst a mass of foam. Maggie is then visited by two strange men who, when they open their mouths wide, fill the room with a toxic gas.

With the pressure dropping in the impeller, The Doctor tries to convince Robson that he has heard strange movements in the pipe but Robson dismisses it as malfunctioning machinery and so refuses all appeals to shut down the pipe. Then Harris arrives and tells him what has happened to his wife. Robson agrees to put The Doctor, Jamie and Victoria into Harris' custody for an hour. However, when they arrive at the Harris’ living quarters they find Maggie in a coma and the apartment filled with gas. When Victoria finds a clump of the seaweed by the bed The Doctor carefully places it into a plastic bag and returns, with Victoria, to the TARDIS. After running some tests he discovers that the seaweed is responsible for the toxic gas. The tests also confirm his suspicions that the weed is alive. After showing Victoria an illustration of a legendary creature witnessed in the eighteenth century she confirms that she saw the same creature when she was trapped in the oxygen supply room.

Frank Harris with The Doctor and Victoria

While they have been away Robson has become more unstable, accusing Harris of conspiring against him. After being attacked in his own quarters he is then seen leaving the complex. Maggie has also disappeared – she is last seen covered in a weed-like growth striding into the sea and disappearing below the surface. Then it is discovered that there is more of the foam and seaweed in the impeller shaft. With the need to reinstate gas supplies to the country Harris finds himself under a lot of pressure. This is increased when helicopters, which had been sent to fly over the rigs, report back that each is covered in weed and foam and that there has been no sign of life.

When The Doctor arrives back he warns Harris that Robson is probably under the control of the weed creature. He also believes that one of the rigs must have sucked up a parasitic form of seaweed and that it is now using the rigs as a bridgehead to spread onto the mainland and, if not stopped, eventually all over the world. To prevent this they must find a way of destroying the creature's nerve centre.

After an attempt to pump pure oxygen down the impeller shaft fails the control room is suddenly swamped by foam - inside which is the seaweed creature. In the confusion Jamie suddenly realises that Victoria is missing. It is soon discovered that she has been kidnapped by Robson, who has left the complex in a helicopter. The Doctor contacts Robson by radio and tries to reason with him, but Robson tells him that if he wants Victoria back he must follow them. The Doctor has little choice and so after a hair-raising flight in another helicopter, flown by The Doctor, he and Jamie arrive on one of the rigs. Surrounded by foam and weed creatures The Doctor confronts a rapidly mutating Robson allowing Jamie to free Victoria. However, when one of the seaweed creatures advances on The Doctor, Victoria can’t help herself but to scream. To everyone’s surprise the creature pulls back. Realising the affect Victoria’s scream has had on the creature The Doctor tells Victoria to keep screaming while they make their getaway.

Robson

With this valuable knowledge The Doctor, back at the refinery, makes plans to kill the weed by recording Victoria’s screams and then channelling and amplifying them, through the pipes, to the creature's nerve centre on the control rig. His plan works and the seaweed retreats. With the nerve centre destroyed and all the infected humans returned to normal, The Doctor make plans to depart. However, Jamie is shocked when Victoria announces to them both that she has had enough adventures and has decided to stay and live with Frank and Maggie Harris.

The Doctor is saddened by Victoria’s announcement but understands her decision to settle down to a quieter life. Jamie though is distraught. The Doctor and Jamie decide to stay another day to check she is sure. When they realise that she is The Doctor and Jamie depart in the TARDIS leaving Victoria on the beach waving goodbye to them both.