The TARDIS materialises on the North Sea and the Doctor, Jamie and Victoria use a rubber dinghy to reach the shore. They find a pipeline running up the beach and the Doctor uses a sonic screwdriver to loosen the screws on an inspection plate. The Doctor believes he can hear a strange beating sound coming from the pipe. 

The three travellers are knocked out with tranquilliser darts and awake to find themselves in a large control room inside a North Sea Gas refinery. John Robson (Victor Madden) wants them to explain why they were tampering with the pipeline and has Frank Harris (Roy Spencer) lock them up in one of the cabins. Robson is more concerned by the loss of contact with one of the drilling rigs at sea. 

The communications officer, Price (Graham Leaman), finally contacts Mick Carney (John Garvin) on Rig D, but Carney is acting strangely. Contact is abruptly terminated by the rig. 

Harris asks Robson to consider shutting down the pipeline as he has gathered considerable evidence about the pressure variations that have been occurring. Harris does not have the papers to prove his point, however, and so asks his wife, Maggie (June Murphy), to bring them from their living quarters. 

As Maggie searches for the papers she is stung by a clump of seaweed and calls her husband who leaves to tend to her. 

The Doctor, Jamie and Victoria, who have escaped from their makeshift cell, overhear Van Lutyens (John Abineri) accuse Robson of damaging the crew's morale. The Doctor decides that he and Jamie should investigate the impeller shaft, whilst Victoria returns to the cabin. She hides in the oxygen supply room as a figure opens the ventilation duct allowing foam to fill the room. Amongst the foam is a moving mass of seaweed. 

The Doctor and Jamie hear Victoria's scream. They race to the cabin and rescue their friend as the monster retreats. Robson and van Lutyens arrive soon after but dismiss Victoria's story as hysteria, although van Lutyens detects toxic gas in the air. 

Harris tends to his wife, who has become very drowsy. She tells her husband about the seaweed that stung her, but her voice trails off in mid-sentence. Harris decides to fetch the Doctor. When he has gone Maggie rises from the bed and opens the door to the balcony where seaweed is growing amongst a mass of foam. 

The Chief Engineer (Hubert Rees) reports from the impeller area informing Robson that the pressure has dropped once more. The Doctor tries to convince Robson that he has heard strange movements in the pipe but Robson dismisses it as malfunctioning machinery, refusing both the Doctor's and van Lutyens' appeals to shut down the pipe. 

Harris arrives and demands that Robson allow the Doctor to leave with him to check on Maggie. Robson agrees to put the Doctor into Harris' custody for an hour. Back at their quarters, Maggie receives an unexpected call from two men, Mr Oak (John Gill) and Mr Quill (Bill Burridge). They open the patio door and the seaweed creature enters the apartment. They then overcome Maggie with a gas expelled from their mouths. When Harris arrives he finds his wife in a coma and the apartment filled with gas. He tells the Doctor about the stinging seaweed and Victoria finds a clump by the bed. Harris leaves to fetch help from the medi-centre. The Doctor puts the seaweed into a plastic bag and returns to the TARDIS, leaving Maggie on the bed. She now has a weed-like growth spreading across her arm. 

In the TARDIS the Doctor runs tests and discovers that the seaweed emits toxic gases. The Doctor also confirms his suspicions that the weed is alive. He shows Victoria an illustration of a legendary creature witnessed in the eighteenth century. Victoria confirms that she saw the same creature in the storeroom. The three travellers return to the refinery complex. 

Robson has become more unstable. He accuses Harris of conspiring with van Lutyens then retreats to his cabin. Van Lutyens asks Price to contact the company's senior manager at the Hague. 

In his cabin, Robson tries to relax. Outside, Mr Oak causes foam, gas and weed to flood into the room. Robson cries out and is rescued by Harris. Van Lutyens arrives too late to see the creature but realises that it can strike at them from any vent in the complex. Robson has run off and the guards are told to look out for him. Harris decides to try to contact Megan Jones (Margaret John) at the company's London board. 

The Doctor, Jamie and Victoria arrive at Harris' quarters but Maggie has gone. They are attacked by weed and foam and only narrowly escape. 

Harris reveals that he has arranged for Megan Jones to come to the plant but is unsure how she will react as Robson is a friend of hers. The Doctor warns the assembled technicians that they are dealing with a sentient seaweed creature. 

On the shore Maggie strides into the sea until her head disappears below the surface. 

Harris searches the beach for his wife but finds Robson instead. Harris backs away when he sees the detached look on the man's face. Van Lutyens descends into the impeller shaft and is soon engulfed by a sea of foam and seaweed. 

The Doctor and Jamie follow van Lutyens down into the shaft. In the communications room Harris is informed that Megan Jones has arrived and he goes to speak to her, leaving Victoria alone with Quill and Oak. 

Jones tells Harris that they must restart production at once as the Southern Regions are crying out for gas supplies. She wants teams sent to the stricken rigs and demands to speak to Robson. 

The Doctor and Jamie find van Lutyens' torch but no other trace of the man. The foam suddenly advances down the shaft towards them and they quickly climb a ladder back into the impeller control area, which is deserted. 

Helicopters are ordered to over-fly the rigs. They report back that each is covered in weed and foam with no signs of life. Jones is astonished when Harris calls for the evacuation of the control rig and the destruction of the other rigs. At that moment Robson returns, crying that the rigs mustn't be destroyed. He then runs away again. 

The Doctor enters and tells Harris that they were unable to find any trace of van Lutyens. He warns Harris that Robson is probably under the control of the creature. 

Jamie searches the refinery complex for Victoria whilst the Doctor explains that the creature's intelligence is the sum of the stolen knowledge from the people it has absorbed. Price contacts the control rig and sees Chief Baxter (Richard Mayes) frantically telling them that he is surrounded before being engulfed by the foam. Contact is broken with the rig. 

Jamie finally finds Victoria in the pipe room and she warns him about Oak and Quill. The Doctor tells the group in the control room that he believes the weed intends to use the rigs as a bridgehead to spread onto the mainland and eventually all over the world. They must find a way of destroying the creature's nerve centre. 

Price is now unable to raise any of the remaining rigs. The Doctor realises that the weed may find pure oxygen toxic but he is overheard by Oak and Quill who immediately go to the oxygen store and empty all the cylinders. 

Harris discovers that the oxygen supply has been sabotaged and realises they have a spy in their midst. When he begins to check the fingerprints of the personnel, Oak and Quill try and escape, chased by Jamie. Jamie catches Quill, who spits toxic gas at his face. Victoria screams and Jamie manages to land a punch. Quill is knocked out but the Doctor is doubtful that it was Jamie's punch that caused this. 

The creature is now rapidly filling the control room which the Chief Engineer seals off. Jamie suddenly realises that Victoria is missing - she has been kidnapped by Robson, who has taken 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 but to agree, secretly hoping that he will be taken to the parasite's nerve centre. 

The Doctor and Jamie board another helicopter and follow the first one to the control rig where they are lowered by ladder. On the rig, the Doctor finds himself surrounded by foam and weed creatures, facing the rapidly mutating Robson. 

Jamie manages to free Victoria whilst Robson tells the Doctor that he will aid the weed in the conquest of the planet. The creature advances on the Doctor but pulls back when Victoria screams. The Doctor tells Victoria to keep screaming. With the creatures crippled the friends make their getaway. 

Back on the mainland, the Doctor plans to kill the weed by channelling amplified screams to the creature's nerve centre on the control rig. Victoria's screams are recorded on tape and the Doctor sets speakers up facing the weed flooding into the control centre from the impeller shaft. The plan works and the seaweed retreats. With the nerve centre destroyed all the infected humans are returned to normal. 

When the time comes to leave, Victoria elects instead to stay with the Harrises. She watches as Jamie and the Doctor row out to the TARDIS and disappear. Jamie tells the Doctor he doesn't care where they end up next as he watches the image of Victoria get smaller and smaller on the viewscreen.

_{Synopsis from Doctor Who: The Second Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}