The TARDIS picks up a distress call on its Mark III Emergency Transceiver and brings the Doctor and Romana to the planet Chloris to investigate. Chloris is a lush and verdant world but has a chronic shortage of all metals, resulting in a band of scruffy thieves organising raids on the palace of the planet's ruler, Lady Adrasta, to steal whatever metal they can get their hands on. Adrasta keeps order with her wolfweeds and guards and controls the planet's supply of metal.

The Doctor discovers that the distress signal is actually coming from a large eggshell-like structure in the forest at the Place of Death, but before he can investigate further he is taken by Adrasta's guards and sentenced to be thrown into the Pit - a fate that befalls all who oppose the Lady.

The Doctor manages to cling to the sides of the Pit when he is thrown in and makes his way safely to the bottom. There he meets Organon, formally Adrasta's astrologer, who fell foul of the Lady when his predictions did not come true. The Pit is also home to an immense green globular creature which oozes through the passages and suffocates those who are fed to it.

The Doctor realises that this monster is not an unthinking killer, but an ambassador from the planet Tythonus. Tythonus has a lack of chlorophyll but has much metal and the Tythonians hoped to trade with Chloris. Unfortunately the first person their ambassador Erato met was Adrasta who realised that if more metal was brought to her planet then her monopoly on it would be broken. Instead she took Erato's communicator device and trapped him in the Pit.

Adrasta is trapped and killed by Erato in the Pit. The bandits, who were robbing Adrasta's palace while she was otherwise occupied, come across Erato's communicator. It exerts an influence over them when they touch it and they are compelled to carry it to the Tythonian. Erato then tells the Doctor that his own people have already retaliated for his imprisonment by setting a neutron star on course for Chloris. 

The Doctor takes Erato and the TARDIS out into space, where the Tythonian spins an aluminium shell around the star and pulls it off course, thus saving Chloris.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}