This story marks the final appearance of companion Victoria (Deborah Watling) and the first appearance of the sonic screwdriver. None of the episodes survive, wiped in 1974, though there are plans to produce an animated version in 2020.
The TARDIS lands in the sea off the Eastern coast of England. On a nearby beach, covered in seafoam, the Doctor uses his sonic screwdriver to examine a Eurosea gas-pipe, and hears what he thinks is a heartbeat within. Naturally, the time-travellers are arrested for trespassing and locked up. Robson (Victor Madden), head of a North Sea pumping operation, and his scientist Frank Harris (Roy Spencer) are worried about Rig D, which they can no longer contact. The Doctor suspects there is a sea creature of some sort in the pipes.
Harris’ wife Maggie (June Murphy) fetches a file for him from his desk, is pricked by a sharp strand of seaweed and falls ill. She is attacked by two technicians who have already been infected by the seaweed, with long green tendrils on their bodies.
The Doctor, freed to help, brings a sample of the seaweed to the lab. In water it grows quickly and becomes malevolent. A creature, accompanied by a powerful heartbeat, tries to break into the control centre through an air-vent. Gradually, the seafoam takes over the labs and the Harris house. Harris and his wife are being controlled. Victoria, of course, is kidnapped and taken to a drilling rig. Robson is transformed into a seaweed creature. Victoria screams and the creature recoils.
The Doctor now knows that the monster is sensitive to high-pitched sounds, so he records Victoria’s screams to use as a weapon. After all, she has done a great deal of screaming in the last seven stories and is in fine voice. This works, and the creature is dissipated, its victims restored. But Victoria, tired of being frightened all the time, decides to leave the TARDIS and live with the Harrises. The Doctor and the heartbroken Jamie leave in the TARDIS as Victoria watches from the beach.
In her stint as a TARDIS crewmember, Victoria picked up a few skills, such as basic nursing and lock-picking, but most of her time was spent screaming in terror. She began as a proper Victorian young lady and was rather shocked by the Star-Trek-like miniskirts worn at Britannicus base, in The Ice Warriors, but as the Sixties wore on her skirts became considerably shorter. I’m sure this was not lost on the primary viewers—adolescent boys. At least she got to do a proper farewell scene, unlike several written-out characters.