The Doctor is in a boat off the coast of Australia. He has heard reports of a Southwell Park mermaid, and now he has met her. He gives her some food and talks with her, marvelling that she can survive in the sea.
Meanwhile, Chris is on the beach, which is crowded with people who have been drawn to the sea. He spots a dead body and calls Councillor Nguyen, but while he talks with her, he sees several UFOs. He takes a quick photo of them.
Later, the Doctor and Chris meet with Nguyen. The Doctor tells her about the mermaid. She mentions illegal drugs and a bacteria used to kill them, and the Doctor thinks the bacteria might be what's affecting the people. Later, alone, the Doctor and Chris discuss the UFOs. The Doctor says they are horseshoe crabs that have evolved into intelligence. They can also fly.
The Doctor and Chris meet the mermaid again. She lets the Doctor take a blood sample, but then she becomes suspicious and swims away. The Doctor accuses Nguyen of engineering the bacteria to affect the people.
Later he realises the genetic programming is too sophisticated. He deduces that the humans' genes had been altered long ago, and all the bacteria did was turn on the altered genes. He thinks it's possible that Nguyen and the horseshoe crabs are working together.
The Doctor realises that Nguyen will try to capture the mermaid. He takes a boat to sea while Chris stays on the beach and tries to keep the people out of the water. Chris sees that the horseshoe crabs are herding people back onto the beach from the water.
The Doctor's boat catches up with Nguyen and her thugs. He tells Nguyen that her bacteria turned off the gene that was keeping the altered genes dormant. He tries to stop her but when he falls into the sea, her thugs throw a net on him. He is sinking fast, but half a dozen horseshoe crabs bring him back to the surface. The mermaid is nearby, and tells the Doctor that she knows he is on their side. The horseshoe crabs bring him back to shore and herd Nguyen's boat back to shore.
- Horseshoe crabs are an advanced species.
- The Doctor carries a pocket knife.
- The Doctor gives the mermaid baked beans and mineral water. Later he gives her Chris' secret stash of chocolate bars.
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