At first glance, Erato is alarmingly large, scary and blobby. This, however, is the unenlightened view. It is true that Erato is a sizeable organism - his enormous body mass is one giant brain with various-sized appendages, with a skin like a cerebral membrane. It is also true that Erato glows an unearthly green color, from the chlorophyll flowing through his veins. And it is true that he makes a hungry, growling sound - but, having no eyes or larynx or mouth or telepathic ability, he is conscious of things around him yet cannot directly communicate with other living organisms in any other way. (Additionally, Erato doesn't like heat, but that really isn't very scary.)
In fact, Erato is the Tythonian High Ambassador from the planet Tythonus. Rather than people, Tythonians ingest chlorophyll and mineral salts, but while Tythonus is overabundant in metallic ores and minerals, its chorophyll supply is severely lacking. Having learned that the planet Chloris suffered from exactly the opposite problem, Erato embarked on a trading mission to the planet. Unfortunately, the first person he met on the planet was the Lady Adrasta, who spurned his offer of metal. Preferring to retain her power rather than improve the welfare of her people, Adrasta tipped Erato into a disused mine and sealed off all the entrances. Here he stayed for fifteen years (which was really a rather short period of time, given the average Tythonian's lifespan of 40,000 years).
Adrasta also took possession of Erato's pentagonal shield, the device he used to communicate. Because it functioned by borrowing the larynx of any person who placed their hand on the central protrusion of the shield, Erato was used to communicating through close physical contact. Deprived of company and desperate to talk with any visitors to the Pit, Erato attempted to communicate but ended up accidentally crushing them.
Erato has the ability to spin metals, like aluminum, cadmium and iron, from his body. He can weave these metals into a shell, creating blockages in passageways or even a spacecraft. Erato's ship, shaped much like a metal egg and equipped with a proton drive, remained on the surface of the planet where it had landed. Erato had been able to carry many of its vital components with him into the Pit, but the distress beacon in the shell remained active, transmitting a signal on very low-frequency wavelengths. When the Tythonians received the signal and found out that Erato had been imprisoned, they launched a neutron star at Chloris' sun. Fortunately, shortly before its arrival, the Doctor and Romana also intercepted Erato's distress signal and came to investigate.
The Doctor was able to hoist Erato out of the pit; once free, he smothered Adrasta and chowed down on her Wolfweeds - the first solid meal he had eaten in fifteen years. With the Doctor's help, he wove a metal shell around the neutron star and it was diverted harmlessly into deep space. With Adrasta gone and Erato free, the way was paved for a new relationship between Chloris and Tythonus, and a draft contract for trade agreement between the two planets was finally established.
