The planet Chloris is very fertile, but metal is in short supply and has therefore become extremely valuable. A huge creature, with most unusual physical properties, arrives from an alien planet which can provide Chloris with metal from its own unlimited supplies, in exchange for chlorophyll. However, the ruthless Lady Adrasta has been able to exploit the shortage of metal to her own advantage, and has no wish to see the situation change.

The Doctor and Romana land on Chloris just as the creature's alien masters begin to lose patience over their ambassador's long absence. The action the aliens decide to take will have devastating consequences for Chloris, unless something is done to prevent it