'The Doctor and Chris investigate reports of UFO sightings on a 21st-century Australian beach, but are diverted by reports of a mermaid sighting. The Doctor eventually tracks down the mermaid, and learns that she's a human who has somehow been biologically adapted to survive in the sea. Chris spots the UFOs moving about the body of a drowned swimmer and takes a photograph of them, and the Doctor identifies them as flying horseshoe crabs. The crabs are the oldest living things on Earth, and some have evolved into sentient beings who are hiding in plain sight amongst the human race; in 200 years' time, humanity will find evidence of their existence and name them as the unknown quantity, the X. The Doctor learns that many of the people at the beach have been drawn here by some powerful compulsion, and that they all come from an area seeded with a genetically engineered virus designed to destroy the plants grown by an illegal drug ring. At first, the Doctor suspects that the authorities and the X are in league and are experimenting upon the Australian citizens, but he soon realises that what's happening here is in fact an accident. The X seeded human DNA with the "mermaid" genes long ago, for reasons of their own, and the human genetic engineering has accidentally activated the recessive genes too early. The Doctor realises that Councillor Nguyen, who has also been investigating the mermaid sightings, intends to capture her and conduct experiments on her in order to increase Australia's understanding of genetic engineering and help to make it a world power; he tries to stop her, but the X get there first and rescue the mermaid. More people are entering the water, and the mermaid assures the Doctor that the X will take care of them, as they consider this accident their responsibility. The X return the Doctor to the shoreline and leave him there, understanding the nature of the problem but unable to do anything else to solve it.'

[The Doctor is the Seventh Doctor.]

Source: drwhoguide.com (http://www.drwhoguide.com/whotrip10.htm). Sighted 20/5/11