The use of an MK3 Emergency transceiver on the TARDIS identifies a distress signal and brings the craft to the lush jungle world of Chloris, where metal in all forms is a rare and prized commodity. The Doctor, Romana and K-9 venture out to discover the remains of an enormous egg in the jungle, and when they meet the inhabitants they find a matriarchy ruled through fear by the icy and callous Lady Adrasta. Without metal to make the tools needed to keep the jungle under control, lush plant life dominates. The Lady Adrasta controls the planet's very last metal mine, holding on to power through the Huntsmen and the Wolfweeds. Her throne room contains an array of metal including a shield patterned in the same way as the remnants of the shell. She mentions the Creature which dwells in a deep pit on Chloris.
Romana has meanwhile been captured by a party of scavengers, ever keen to find and hoard more metal, and they are particularly impressed by the possibilities of K9. The robot enables her escape and she is briefly reunited with the Doctor before he leaps into the Pit himself, determined to get to the bottom of the mystery and the Pit. Within the Pit he encounters Organon, an astrologer thrown there by Adrasta some time earlier, and then comes face to face with the Creature: indeed, the vast shapeless blob rolls over him. The Doctor calculates it is not, however, dangerous, and is fascinated to note the Creature produces metal from within itself. It also forms a tentacle and draws a picture which the Doctor recognises as the shield from Adrasta’s throne room. Lady Adrasta, her lady-in-waiting Karela, the Huntsman, his wolfweeds, and some guards, enter the Pit from a secondary entrance and make their way to the Doctor, Organon and the Creature.
The scavengers have meanwhile raided the throne room for booty, including the alien shield. It exerts influence over two of them, who take it down into the Pit and place it on the Creature. It turns out that the shield is a communication device. Erato, as the Creature is named, is the Tythonian ambassador to Chloris and came to negotiate a treaty exchanging metal for chlorophyll fifteen years earlier. Its craft was the vast egg found in the jungle. However, Adrasta realised her power was dependent on the control of the planet’s metal supply and so imprisoned Erato to maintain her status. The Huntsman orders the Wolfweeds to detain Adrasta as Erato takes his revenge, smothering the evil ruler and dissolving her in seconds. The Doctor makes arrangements to have Erato lifted from the Pit. Meanwhile, Adrasta’s sidekick, Karela, attempts to capitalise on the situation and seize power herself—but with the help of K9 the Doctor brings it to nought.
The Doctor has rescued the Tythonian just in time – it seems Tythonus has declared war on Chloris over the missing ambassador, and has despatched a neutron star to collide with Chloris’ star and destroy the system. It is due to collide within the next twenty-four hours. Working against the odds, Erato, from his reconstituted spacecraft, weaves a metal covering around the star, enabling the Doctor, using the TARDIS gravity beam, to draw the star off course and neutralise the danger. The Doctor’s last act on Chloris is to push the Huntsman, now one of the de facto rulers, toward a mutually beneficial trade agreement with Erato and the Tythonians.