The TARDIS detects a distress signal and materializes in the Lush jungle of the planet Chloris. Metal on Chloris is rare and valuable. The Doctor and Romana step out and discover a giant egg, and they find a ruling matriarchy run by the icy and callous Lady Adrasta (Myra Francis), who controls all the mining through her Huntsmen and their Wolfweeds. Her throne room is made of metal and she mentions a creature in a pit.
Romana is captured by scavengers, who are extremely impressed by K9. The robot helps her escape and she catches up with the Doctor, who proceeds to leap into the pit. He meets Organon (Geoffrey Bayldon), an astrologer thrown there by Adrasta, and then the creature, a huge shapeless blob. It seems harmless and produces metal. It forms a tentacle and draws a picture of the shield in Adrasta’s throne room. Lady Adrasta, her lady-in-waiting Karela (Eileen Way), the Huntsman (David Telfer), and his Wolfweeds enter the pit.
The scavengers raid the throne room and take the alien shield. It takes over their minds, forces them to bring it to the pit and place it on the creature, where it turns out to be a communication device. Erato, the creature, is the Tythonian ambassador, having come to Chloris to negotiate a metal for chlorophyll trade fifteen years ago. The giant egg in the jungle is its spaceship. Adrasta, not wanting to lose control over metal, imprisoned the ambassador in the pit.
The Huntsman rebels and sets the Wolfweeds on Adrasta. Erato rolls over them, devours the Wolfweeds, and leaves Adrasta a corpse. The Doctor orders Erato lifted from the pit. Though Karela attempts to seize power, the Doctor frustrates her plans with the help of K9. Tythonus has declared war on Chloris because of the missing ambassador, sending a neutron star to destroy the system. Erato weaves a metal covering around the star, allowing the Doctor, with the gravity beam of the TARDIS, to draw it off course, Before leaving, the Doctor encourages the Huntsman, now a ruler, to reach a trade-deal with Erato and the Tythonians.
The Doctor’s plan to weave a shell of aluminum around the neutron star has been scoffed at, but the idea came from the Institute of Astronomy in Cambridge. The big blob Erato had, in the beginning, a phallic-shaped proboscis, but that was quickly changed. You wonder sometimes if the creature-shop guys do this on purpose to see how far they can get. David Brierly replaced John Leeson as the voice of K9. Reviewers seemed unable to decide if the story was a spoof of bad science-fiction or just bad science-fiction. In either case, it was prime Douglas Adams. This was the first Romana II story filmed and Lalla Ward was evidently not quite comfortable with the role.